I've recreated this report but now used a vanilla client instead of optifine.

An incorrect potion effect is displayed after drinking a shorter, stronger potion and then a longer, weaker potion. The effects are normal, though when the longer one runs out, the display in the inventory shows the stronger potion effect at 0:00 - as you can see in the video.

Would be nice if 1.16 releases with this annoying bug fixed.