The culprit was a broken sound stream. Each frame had a sound stream block, but the sound stream header was missing so every frame was generating garbage audio and stopping playback.

A summary of the commands I used to fix this without deleting all 1487 of these tags in the ffdec GUI:

ffdec -swf2xml "[TVComrade] Lady Bow vs Anon.swf" out.xml

(memory is hazy on the following one because I deleted the xml and can't check what a sound stream tag looked like without waiting for another xml conversion)
sed -e "s/<item type=\"SoundStreamBlock[^>]*>//" out.xml > nosound.xml

ffdec -xml2swf nosound.xml asdf.swf

ffdec -compress zlib asdf.swf "[TVComrade] Lady Bow vs Anon (broken sound removed).swf"

The visuals remain glitchy (penis has an incorrect bounding box so it doesn't get cleared properly and leaves trails) but at least it plays uninterrupted. It's also missing the music obviously. I wonder if it was like this in the Newgrounds release??? I will not bother to check.

[TVComrade] Lady Bow vs Anon (broken sound removed).swf (3.23 MiB) 1920x1080, Compressed (Deflate). 1487 frames, 30 fps (00:50). Ver30, AS1/AS2. Network access: No. Text: No. Bitmaps: Yes. Audio: Yes. Video: No. <METADATA> [find in archive]