This seems to have something to do with spawn chunks, because I'm usually unable to reproduce it in them and I'm usually able to reproduce it outside of the spawn chunks. But when leaving or entering the spawn chunks, it doesn't always start or stop happening.
Leaving the blocks while having the sprint key pressed starts sprinting, entering blocks while sprint-flying slows you down again.
Linked issues
duplicates 1
Comments 3
Could be, but then why can I sometimes not reproduce it? Does it depend on my height inside the block when I enter the blocks or when I start sprinting?
This is explained in MC-147770.