When a fluid is placed, there is a certain delay until it flows. If the fluid is removed, when the game tries to make it flow, it will realize that the fluid has been removed, and cancel the spread of the fluid to adjacent blocks. This works as expected. However, if the fluid is quickly placed again after it has been removed, but before it would have flowed, it will flow at the time determined by the initial placement of the fluid, rather than the most recent placement. This issue is most easily reproduceable with lava, since it is slower, but some high-speed redstone devices using water could also expose the issue.

I have attached a recording of this issue occurring in 21w44a.