I recently posted a HUDless webm of artist falls-down-steps' Beat Banger mod. However, it was deleted for being a "screencap" (https://e621.net/posts/2841410?q=mao_mao%3A_heroes_of_pure_heart ). Meanwhile, other Beat Banger videos go untouched despite also being recordings of gameplay since, well, Beat Banger is a game. Why was my post deleted while others stay up? What did I miss by making my post?

You should DM Mairo. He's the one that deleted yours and approvd multiple others, so he's the only one who could tell you what his reasoning is.

Every single goddamn time people ask about deletions and say that there's other posts on the site: always rely on uploading guidelines, not on other approved posts.
There's literal millions of other posts at this point and circumstances you might not have taken into consideration, e.g. grandfathering or accidents. Also I have deleted many Beat Banger mod posts already, even if some of them are up.

uploading_guidelines#bad

Bad things to upload:

• Screen captures: Screenshots from games, still images from movies, video snippets from YouTube, etc.
• This also includes all ripped image files from visual novels and similar games.

Basically any kind of content created from another form of medium is generally deleted as screencap. This also includes stuff like live gameplay recordings, assets being directly ripped from the game, etc. etc. and this includes mods for the game.

General exclusion for this rule has been that if it's content creator themselves that creates something like this, e.g. someone who is creating a game posts showcase of their game, that's generally accepted, however other guidelines like quality requirements, advertising guidelines, etc. are enforced more in these cases.

So in this instance post #2841113 was already on the website and would've been acceptable even if it was screencap, but the overall quality of that post was not meeting the basic quality standards for the site, so was deleted as such. You would have to get the author post their mod in much more representable form before that can be accepted here.

furrin_gok said:
You should DM Mairo. He's the one that deleted yours and approvd multiple others, so he's the only one who could tell you what his reasoning is.

I do always double check situations I'm unsure of with head admin, so at least they should also be aware of how things are handled and why.
I think I did also talk with couple other janitors about rulings with screencaps pretty recently.

furrin_gok said:
You should DM Mairo. He's the one that deleted yours and approvd multiple others, so he's the only one who could tell you what his reasoning is.

I didn't know how, sorry Mairo. The banner with "send message" doesn't show up on mobile so I didn't think it was a feature.

After four uploads, multiple people dmailing me about this (primarily RakushunX and Lord iX) and this forum thread, here's what is now the situation:
Artist did end up also posting this on FA, where they also linked a seperate gameplay video in description, which is weird because it's completely differend from one they posted on twitter. It seemed identical to post #2841410, of course the gameplay would be identical, but in this instance the visuals were frame perfectly identical and the metadata also had exact same timestamps, but the video was transcoded, further decreasing the quality. So technically that would've been acceptable, but it's inferior quality version of something that we had deleted here. One choise was to then maybe restore the exsisting post at this point, however this is highly unusual case of artist taking something from here and then destroying the quality of their own work even furhter.

So I poked our head admin about it and basically said "do it yourself if artist can't" and because the artist themselves has in this instance shared two captured versions of the same gameplay, I guess the screencap could be acceptable in this instance.
The previous upload was visually pretty compressed and 30 FPS (and VP8/Vorbis with 500kbps audio for some reason?), where the Beat Banger supports arbitary framerates with some effects being capped at 60 FPS, so here's post with proper quality with full 60 FPS and much higher visual fidelity:
post #2852731

So again, third party screencaptures from stuff like games and mods are always subject for deletion, we can accept screencaps from authords themselves to showcase their own works. Just in this instance it seems like the artist was unable to create that showcase themselves to degree were they simply took e621 users file to share, so if it's down to that, might as well make it perfect.