Pics of damaged sick Sans. hope this works...

Here is a link that might be useful: Sans browning tips

Jack,
All plants loose leaves at some time. If it's just a few, your plants are ok. Simply cut out the bad sections. New ones will grow in. You just changed the envelopment they were used to. Give them a chance. Sounds like you know what you are doing. Base rot is a different thing. If that happens. The you will have to repot and save what you can. And mushy or wet spots have to go.

From the pic, it's possible that one leaf did get burned by the heat of the fireplace. If the other leaves are still fine, firm, I wouldn't worry but definitely keep an eye out for any other signs of stress/damage.

From what you wrote, this is what concerned me, "Pretty dry, feeling ever so slight dampness if you put your finger about 2 inches in the soil." If it's moist below, more water is not needed until the soil has dried completely to the bottom of the pot. To me, this would be a pot that's still quite moist, assuming it's a foot tall or so, wouldn't consider watering it.

Leaves can be damaged at the tips in the case of frost bite, which takes a couple weeks to show up after it happens. Leaves can also show damaged tips if there is a problem with the roots. There's not enough info here to know which is going on, if either.

" I filled in the gap with miracle grow cactus mix but they are basically in the same soil." When a plant is potted up instead of repotted, there can be problems, especially when 2 different kinds/textures of soil are used.

In this particular case, since you mentioned the new outer soil is a more chunky, airy type, the outer soil may be drying very quickly while the older, inner ball may be staying really wet.

Sans can keep individual leaves for many years, so the loss of one is definitely cause for investigation, but not necessarily concern if it turns out to be something like an isolated past heat or cold exposure.

Assuming it's not rotting now, when practical, I would take the troubled plant out of its' pot, completely get rid of all of the old soil, then repot with all new soil. You'll probably find several individuals, connected to each other by big roots called rhizomes. It's fine to separate any that each have their own root systems, so you can give each section as much space in the pot as possible. If you find anything mushy/slimy under the soil line, cut that off, then let the the good parts dry overnight before repotting. If the soil is moist when you put the plant(s) back in, as bags of stuff almost always are, don't water it in, or at all until it is dry.

When Sans (or any plants) are not in potting soil, but in a more chunky, airy mix, the risk of rotting roots is extremely low, and drying to such a severe degree isn't necessary.

Thanks Stush2049 That is encouraging :) and for all the suggestions. I could not figure what I was doing wrong.. I researched their care. Most things said they were almost impossible to kill. Which is why I purchased them lol

After looking into the plant I realized there were 2 babies coming out at the edge of the soil with the off white roots kind of exposed. The center of one pot was so crowded I couldn't even see in there. I was very curious to see what was going on underneath.

So….I slid the plant out of the pot, into a basin to see. First of all, I was glad to notice the soil felt a bit on the warmer side, which I was happy about considering I was worried about it getting cold in their pot, with the weather we’ve been having and all the roots on both plants were healthy. The soil seemed dryer down there than I thought. Most of he soil pretty much just fell of the plant when I took it out. While doing this, I found one of the dying leaves was attached to the 2 babies. It looked like it was just stuck in the soil with some roots on the bottom and 2 babies branching off from it. Not sure if this is what this plant does but I’m thinking the leaf served its purpose of creating new life and was now forcing most of the energy into the babies which is why it was dying. (my theory anyway ☺ ) One of the other dying leaves did have babies attached underground as well that did not come to the surface yet. I also noticed that one of the plants toward the center was extremely crowded where a one of the leaves were browning and had drying tips. I figured I’m still under the Lowes plant guarantee so what the heck! I separated the plant a bit so it had more air circulation and light to those center leaves, cut the babies and their roots from the leaf that was stuck in the soil and repotted them in and everything else. It looks much better separated rather than all crowded in the pot. Hopefully it will be fine now. After all this trimming and replanting, I had some of those roots lying there with the finer hair like roots attached. I was about to throw them out and decided I would experiment and plant them to see if I get anything. We shall see how it goes. ☺

Very glad to hear such goodness.
Stush

Browning leaf-tips are nearly always an indication of dying root-tips, as above so below. Mixes like MG tend to hold excess moisture, and also tend to hold lots of salts when the mix is watered in sips during the Winter. I would recommend a re-potting into a properly draining and porous mix this Summer when the growth is strongest.

Josh