then YES this is a really infuriating thing. mapping extremely long songs with this issue is a really dreaded thing for me.

maybe my explanation and assumptions are incomplete and i don't know technical things so i'm open for being told at.

actually, from what i presume...
this happens because when some developer associates creating a new timing section from a mapper viewpoint, the beats affected have to adjusted (maybe). and its really painful to have to adjust an entire section. so what happens is the beats adjust to the new BPM you are currently working on.

two issues:
1. this relies on sections of a map.
nowadays, however, we also make maps which are not well timed by the artist at all. the game is not supposed to have those songs. its a rhythm game. therefore you're screwed the moment you decide to map an irregularly timed bpm.

does this not apply to you? then for the general case...

2. for some odd reason, it affects beats in sections before and after it. (maybe this is just my case but mhmm...)
though when you go backwards it updates to use the timing of that specific section, the memory (??) of the other red lines still exist in the game. so the circles from the prev and the next timing section are still some sort of "in the scope" of the new timing section. and therefore those circles are also adjusted.

so when circles are time-adjusted, it sometimes goes out of bounds (negative time) of the editor scope. and so, once you save, it ends up saved at a negative number. osu! cannot handle that and will ignore it in the interface, and since its "not there" anymore, osu! will adjust by deleting those circles. or so i think.

this issue has made me map the first song of a certain marathon experiment i made of songs which, unfortunately, aren't well timed. hated that. i had to recall from scratch again and again.

oh ok whoops sowwy :>