i can concur what Tara is experiencing, though i've not uploaded anything recently...

my experience, at least fairly recently, is that i upload an image with my own thumbnail that i made. roughly two days later i go to my gallery, and somehow my handmade thumbnail had disappeared, or had been replaced actually, by the full image, or what would appear had i opted for an auto-generated thumbnail. so to get back what i WANTED to show, i had to re-upload my own thumb again, but i had to do it twice before it stuck/took.

another issue is that if i simply edit words that i've put under an image [ONLY words, or maybe this happens simply because of the edit mode itself], even though i keep the 'keep this thumb' option ticked, after editing, i get the auto-gen thumb instead [i've left an image in my gallery in this particular stated for comparison, as i too, use/prefer uniform thumbnails throughout my gallery].

i've also noticed, in the 'what's new' gallery, images that i've previously looked at and know what the thumbnail should look like, suddenly seem to appear to have an auto-gen thumb instead, which has made it difficult to quickly re-identify an image i may want to revisit because the thumbnail no longer appears as what it was, or how i remembered it/what i'm looking for in recall.