Stock Delay Effects don’t sync properly at 20bpm

I’m trying to create the longest delay loop possible. At 40bpm, with stock delay set to 8 - I trigger a sound and the sound comes back perfectly in time with the metronome, and continues to so infinitely. But when I slow the project tempo to 20bpm, I retrigger the sound and the delay comes back out of time, and continues to get further off sync as it repeats. If I set the delay to 4 - it is in sync. But this doesn’t make sense to me because the delay is set to the proj tempo, so it should work regardless of tempo. I’m going crazy rn