Are you using quilt?

im using forge

wow what a quick response!

anyway i'm having the same issue with quilt however i doesn't seem to be happening with fabric
at least for me.

Can you try opening minecraft with forge and the mod and nothing else to see if the issue still persists, i normally do that to test if another mod that is installed is causing issues.

i noticed it only happens when i install certain mods like Effect Tooltips, and Stylish Effect, and maybe other mods too

This is caused by iceberg. Different version of it cause it to either work properly, or to draw that line in different places. Not sure why, or sure about which one needs to be fixed.

I also have found this config option for legendary tooltips:

		# Whether item names in tooltips should have a line under them separating them from the rest of the tooltip.
		name_separator = true

Can we have also something like "name_separator_position" to solve this once and for all?