The fact that it sealed in the water should not have anything to do with whether or not the seal failed.

There may have been a dent or ding in the lid or ring. Another possibility is jam working its way under the lid. This can happen if the headspace is inadequate or if there's a speck of jam that didn't get cleaned off the rim before sealing.

It's all part of the learning curve. See if you can work out which issue is most likely and then just focus on trying to do better with the next batch.

Congratulations on your strawberry jam! Regardless, you'll enjoy it.

Carol

Did you remember to put the lids in simmering water for several minutes before putting them on the jars. You have to do this in order to soften up the rubber so they'll seal tight when they're mashed down on the jar rim. Also, if you aren't careful in wiping off the jar rim prior to seating the lid, you won't get a good seal. These a a couple of my own personal bugaboos that I have to remind myself about every canning season.