If I give a splash potion a blindness effect with a high duration, the game ignores the duration and uses a random time frame somewhere under 5 minutes. When the time runs out, the potion effect disappears and the counter is stuck at 0.
(Non-splash potion supposedly does the same but it counts from 20 minutes. I didn't wait to see what happens there.)
Sometimes (mostly in creative) the duration is fine, but the effect isn't visible.

Other potions (I tried jump boost, but I've seen others in videos) are working fine.