Which did you plant?

When I have grown mositure-loving salvias in humid climates like the gulf coast or the northeast, I have usually mulched with grass clippings because grass clippings are free, suppress weeds well, break down easily, and I think they have some nitrogen as well. For winter, when I am going to cover the plants to protect them frost cold, I always use fallen leaves, preferably oak leaves. I remove the leaves from the plants in the spring, but leave them covering the ground around the plants to keep weeds down. I have also used organic potting soil, compost or similar organic, easily decomposed mulches. In watered gardens or humid climates, like the eastern half of the US or a watered garden in CA, I don't think keeping moisture in the soil is the major reason to use mulch - I think weed control and adding organic matter to the soil are more important. Salvias I'd include in the "moisture loving" category include S. splendes, S elegans, S. guaranitica, S. uliginosa, S. mexicana, S. madrensis, S. 'Purple Majesty', S. coccinea, S. leucantha, S. greggii, S. microphylla, S. 'Waverly', S iodantha, and probably a lot of others. I know many of these are drought tolerant and some are from fairly dry places, but in the garden all of these tolerate or even thrive with ample moisture, and don't seem to suffer from a thick organic mulch. In a garden in the east, you're likely to have moist soil in the summer whether or not you water, and the mulch is valuable to keep weeds out. If I was growing salvias in dry conditions in the west, or trying to grow dry-climate salvias in the east, I would not mulch with grass clippings or leaves, I'd use something like gravel or sand, or simply a 'dust mulch' of cultivated soil to keep weeds down. In this situation, I'd want the soil to dry out after a rain as quickly as possible, and the mulch would slow the drying process down. Salvias in the 'dry soil' category include most California native salvias, which do not tolerate summer soil moisture very well, although some like Salvia clevelandii will grow well with summer moisture, but may not live very long. In a moist soil they might last only a few years, while in the wild, where soil is very dry all summer, they might live for decades.

I personally don't like to use hard-to-degrade organic mulches like shredded bark or wood chips. Most salvias grow better with nutrient rich soil, and I think bark or wood tends to reduce the soil supply of nutrients as those mulches decompose.

Party decomposed hardwood chips have worked fine for me in one situation. I had to use something and the bed was a quarter mile long so it had to be free. A table spoon of Osmocote was put below soil level next to every sage. I don't use woodchips at home, mostly for the reason you state. In the past I've used hardwood bark in some areas but it is a pain during cultivation time because I don't want the chips in the soil, just on it.

And you are right, I'm in the east and I use mulch mainly for weed control. In beds where annual sages and tender sages are grown I use permeable fabric mulch and then lightly cover it leaves. That has really cut down on the amount of weeding each summer and allowed me to bring more ground under cultivation.

In the hot and humid southeastern USA, I prefer hardwood bark, because it does not break down as fast as shredded oak (or better, maple) leaves. The rate of breakdown is almost too fast for leaves, and is good for the bark.

A rule of thumb in synthetic organic chemistry is that reactions accelerate logarithmically by a factor of 10 for every ten degrees. For 20 degrees, the factor is 100. I can see this applying to soil temperatures as well.

I avoid wood (cellulose) chips, because these attract termites.

The humus from bark is ideal for sages in the Carolinas and generates a rich humusy soil as it breaks down.

That is certainly true of high country soils and soils of the far north. Northern muskeg is the extreme example where organic matter accumulates much faster then it can decompose.