The bug
Most GUI sprites either follow or make an attempt to follow what is specified inside of the associated .mcmeta file with respect to the "scaling" parameter. However, for some reason, the icons used for status effects ignore them entirely.
This was recently fixed for empty slots (MC-277486). However, as of 24w44a, mob effects remain affected.
How to reproduce
A resource pack to test this is attached.
The changes made are as follows:
regeneration uses "tile"
poison uses "nine_slice"
hunger uses a sprite which matches the expected outcome of "tile"
wither uses a sprite which matches the expected outcome of "nine_slice"
Expected results
The regeneration effect icon would appear grid-like, similarly to the hunger effect, and the poison effect would be pale red with a thin dark red border, identically to the wither effect.
Actual results
Both the regeneration and poison effects incorrectly have a thick red border.
How to fix
This could be fixed by moving the existing mob_effect texture folder to textures/gui/sprites, and making mob effects reference the gui atlas rather than a dedicated mob_effect atlas.
Can confirm.