Beatmaps with Single-BPM and Multi-BPM timing must be perfectly timed. This means BPM and offset are exactly synchronized with the song.
Unfortunatelly there's no indication about how perfectly it should be timed, because some of mapsets use timing sections with ~5 ms and more which causes playing issue. I suggest to add allowable error for this point of ranking criteria: no more than ~3 ms of error of the synchonization with the music is allowed.
Like it's just addition to this rule to make it more strict and formal to avoid mapsets with unplayable timing settings.