I get this issue sometime with LUI. It is weird. Sometimes I change talents/glyphs with no problems, other times I get message saying I have to disable LUI if I want to perform this action. It is annoying. I just wish it was consistent on whether it will cause a problem or not.
- login without any addons
- in Wow options change the Social-> New whisper from inline to New Tab (if isn't already)
- whisper yourself (or anyone whisper you)
- go change the glyph
- enjoy your taint
As far as I know, it's Blizzard's fault. Just like tons of other things, they say it's not. Like how viewport and similar mods will cause graphic overlay issues in certain places, like Uldum, and there is literally nothing the author can do to make that not happen because Blizzard did something on their end that causes otherwise fine coding to not work properly.
Once upon a time, druids in shapeshift forms that got knocked back would also be "stunned" for a short period of time and I was told numerous times "oh lol clear your cache and WTF folders, it's an addon issue." Bitch getting stunned after a knockback on my druid while in form, but not while not in form, and not on any other character, and all my druid friends have this issue too, is your fucking fault. Trying to feed me bullshit about how an addon could stun my character, but only in shapeshift forms! As if.
Anyway, long story short, either get rid of the addon(s) that cause it, or live with it and hope Blizz gets their end of their shit together.
Like I've said above, it's a different issue causing the same symptoms. The dropdown taint bug (which is what had caused this issue in Cata) has been fixed in 5.0 (or at least it was fixed in the PTR FrameXML - can't check if that change got pulled [for whatever reason] before the patch went live).
UI & AddOns expert | Interface & Macros moderator - My work
That's the dropdown taint bug. I believe that one was fixed during the 5.0 patch, but as I said above, I'm not sure.
This new taint bug is different - it can happen even without any addons (global leak of _ in both secure and insecure Blizzard code, if I remember correctly).
I think that one was fixed. You can /dump issecurevariable("_") to see if its tainted. I can change talents and glyphs still manually tainting it (/run _ = "blah"), and when it does happen and I am unable to change them "_" is usually still secure.