Using /tick to freeze the server and/or run it at a non-default tick rate leads to several issues:

1. Potion particles are created based on the rate at which the player experiences time rather than the game tick rate (MC-267210)
   • FWIW this is inconsistent with enderman particles
   • Although I disagree with the WAI on 267210, I mention it here not so much to get it reviewed but because it would necessarily be part of a comprehensive fix for this issue
2. Potion duration (in UI and potion/tipped arrow item tooltip) is adjusted to account for the tick rate
   • This could be considered intended, but I hope not, since this makes it harder to find the right potion in the creative inventory
   • This causes the potion duration in the UI to misbehave with higher tick rates (MC-266100), although a separate fix for this is possible
3. Potion effects continue counting down while the game is frozen
   • Effects which fade in or out (eg. Darkness) or perform an action periodically (eg. Regeneration) still depend on the tick rate

Expected behavior

1. Potion particles would be created at a fixed rate (eg. 1 particle every 4 game ticks)
2. The potion duration stated in item tooltips would stay the same regardless of the current tick rate. As for the UI, the number would simply count down more slowly at lower tick rates and faster at higher ones.
3. Potion effects would not count down while the game is frozen

Note

I created this ticket so that all the related issues, regardless of whether they've been reported already, can be logically grouped together. I would appreciate a detailed Mojang response such as "Point 1 remains WAI, MC-266100 should be broadened to track Point 2 in its entirety, Point 3 is a confirmed bug and should be tracked in a new ticket. Resolving this as Invalid.". Or, even better, "these are all valid issues and will be fixed together or not at all, so we'll track them here from now on."