CurseForge seems to have some kind of attachment to the vanilla launcher's profile listing. I'll try to explain in detail.
let's say you make a profile in the vanilla launcher, 1.12, and a forge modded 1.16 profile through curse forge.
If you launch the vanilla game, then try to launch an instance from curse forge, it shows the profiles of the vanilla launcher. In this example, it would show the 1.12 profile and not the 1.16.
This also has the same behavior in reverse. If you were to launch the curseforge instance, then open the vanilla launcher, you would find your 1.12 profile gone, and a 1.16 forge profile in it's place, leaving you unable to launch your vanilla-launcher setup.
If the instances are separate they shouldn't have overlap.
Mojang has been making a lot of changes to the launcher lately, and it's likely one of the recent updates changed how profiles are read, and broke something that curseforge previously relied on.
This makes it impossible to effectively launch minecraft more than once while using curse forge.