Hey there! I actually had this happen to one of my violaceas too. But in my case the problem was in the packing. The plant had been taped down into the pot, but the packing material was covering some of the leaves and smooshing them into the wet media. They ended up with "damp rot" or "soft rot," (or whatever the term is) on the leaves that were in contact with the media while traveling. It ended up losing three of its leaves, but the others are doing just fine, and a new leaf is growing now.
This looks like a case of damage in transit. For whatever reason, it looks like the bottom-most leaves on your plant got smooshed during packing or spent too much time in contact with the moist media. I think you have that "soft" rot situation going on here because of physical trauma. The good news is that it won't spread to the newer leaves; it will only affect the ones that were damaged during transit. So once your bottom leaves fall off, you should be okay.
As long as the roots look healthy, which you say the do, the plant should bounce back. It may take a little longer to recover because of the lost leaves, but I don't think what's going on here is a fatal problem. Just make sure to give it lots of TLC.
I hope it works out! Violaceas are so sweet!