Problem/Bug Shulker Hitboxes not properly interacting with portals

Hey

I'm admin on a small SMP, one of the guys made a shulkerfarm in which new shulkers are teleported sideways against nether portals, as soon as their hitboxes expand when they peek and touch the portal they're supposed to be sent trough, though this is not the case
It seems to not be a plug-in issue cuz our SMP has a testingworld without plugs exept for WorldEdit which isn't in the main one
I can't seem to find the issue here, I fixed it with a commandblock for now, pls help

- me