I didn't keep a track of all concerned releases unfortunately, but i import a lot of Bandcamp releases, and as you know i maintain Bandcamp importer, so i just report minor quirks i encountered.
In general, _0 is working but, last summer, i had to patch Bandcamp importer to use _1 because some _0 images were truncated (only when original was a PNG, near the bottom a band of 20 or 30 pixels was missing, https://github.com/murdos/musicbrainz-userscripts/commit/044037ab498445ec0a6f2142777adf11f0ed60b4) while the _1 was correct. I decided to patch as soon i noticed the problem before editors started to upload truncated images (because it wasn't easy to notice), and because uploading a PNG to CAA is still better (even if original image is a jpeg).
I just tested those again, and apparently it was fixed on Bandcamp side, _0 and _1 are identical now.
When it happened, i did tests using different downloaders, and different systems/machines and it was an issue on Bandcamp side for sure (perhaps a web server configuration issue).
About the fact some images _1 not being always PNG it happened few times, but not since months, i have no track of those.
I guess those issues were just temporary, but hard to know if all releases were fixed.