The counter stops tracking at 00:30

https://streamable.com/4pshk2

Thanks boggart, found it. Kills were counting only once per monster spawn spot per session: when a Jester respawned in a spot where you'd already killed one, the game treated the new kill as one it had already counted. That chimney map has about 30 Jester spots, so the count stalled at 28. A refresh cleared the memory, which is why that helped. It's fixed in today's update: reload the game (or press OK on the update notice) and every kill counts again. The 28 you already have are kept. Flash Jump had nothing to do with it.