Hey so We have been putting together custom blocks based off of the ones that ship, and then putting them in sanctioned categories i.e. create a new category - > create block -> clone block in the category. After making around 200 new blocks we wanted to create the thumbnail images. On the first block, we clicked the regenerate thumbnail image and it overwrote every one of our new blocks, as well as removed them all from their specified categories. Fortunately, we were able to pull a backup from yesterday so we saved around 30-40 custom blocks but this is quite the bug.