It seems like webm files above a certain file size aren't being loaded correctly, although I'm not sure what that size limit is.
The images and webms are all downloaded from E621 using the same third-party tool, so they're all named in the same way - a string of letters and numbers.
I created two new directories - one with 46 webms, all renamed to be "webm1.webm", "webm2.webm", etc. - and one with only 12 webms, all under 3 MB (for comparison, the biggest file in the other folder is 95.8 MB). Sometimes, one of the "webm1"s would load, one of the smaller ones, but afterwards it would get stuck on a black screen. The other folder loaded just fine, with it cycling through webms correctly - except for one point where it got stuck on a black screen. I'm not sure what happened there, as the largest file in the folder, 2.99 MB, was one of the first to be displayed in that round.
Things to note: I assume this is intentional, but webm videos ignore the setting for when images should cycle, and *usually* cycle at the end of the video. However, videos shorter than the setting (ex. two second long video, when its set to cycle every ten seconds) will loop until its time to cycle - but in between loops, there's probably half a second of just a black screen. This is especially jarring with very short videos.