So I've been working on designing a slime farm in a test world for a 1.18.2 server and I've gone with a portal design to try to allow as many slime to spawn as possible. From what I've seen in videos on Youtube (any nether portal based farms) the slime should spawn and then immediately go through the portal. That is the complete opposite of what's happening, thanks to tweakeroo I can see that most of the time they just hop around inside the portals for at least a few minutes, if not forever. Using worldedit I spawnproofed well above the needed radius and slabbed every non-obsidian block within range at y=-59 as well as the roof of the farm. It's a triple slime chunk design and goes down to y=-59 with the afk platform at y=-68. I also cheated slightly at first and copy pasted some of the portal blocks, so just in case I manually relit every portal, and the issue persisted. I then removed all non-essential mods (I kept tweakeroo and the carpet mod installed and running), no changes. Finally I ran two 10 minute sessions, one with the above mentioned mods & one in vanilla. The modded test had the carpet mod saying that the farm ran at a rate of ~4126.5 slime/hr while the vanilla test gave me 663 slimeballs over those ten minutes, meaning it ran at a rate of ~3978 slime/hr. I've searched Youtube, Jira and Google and I can't find anything pertaining to slime having a delay timer in relation to travelling through a nether portal. I'm hoping this is a bug and will get fixed soon as otherwise this may also break/have already broken other farm designs, including possibly other portal-based hostile mob farm designs.
Here's a video link showing it occuring:
https://www.youtube.com/watch?v=WY0k4a0Glro