First of thank you for all the work by maintaining the addon!

As for the issues ever since the recent updates to the TWW version Threatplates seems to have some issues with the visibility of nameplates:

[1] Whenever I hide my UI the TP nameplates are no longer hiding like before but instead stay visible (they even enable ones that shouldn’t be visible due to my occluded settings)

[2] When hiding my UI Threatplates just randomly shuffles the nameplates around and puts them on the wrong NPCs. In these two screenshots the only thing I did was hiding/enabling my UI.

UI Hidden:

UI Enabled:

[3] The “Occluded Unit” setting also doesn’t seem to work that well currently (I have set it to 100% transparency, so I don’t see them when occluded). Nameplates now stay visible far longer or don’t disappear at all when they should (before it worked just fine).

I cannot reproduce these issues on Retails. Hiding the UI also hides all nameplates and occluded transparency also does work although I did not test it a lot.

Occluded transparency does not fully work on Classic, but that's a WoW bug and only happens when you target a unit (which does not seem to be the case in the third screenshot).

Hm, unsure right now what could cause this. Did you have any other addons enabled or did you do your tests only with TP enabled?

After you comment I did some further testing on this issue.

First, I disabled all addons and for whatever reason all of the above issues persisted, which would indicate that this could be a Blizzard issue. Though I haven’t heard of anyone else experiencing this, so I continued. I reenabled all addons and disabled them one after another to see if there were any improvements.

When I disabled the “Musician” addon all issues seemed to be resolved, but they would still sometimes resurface again for no apparent reason. I also could temporarily fix the issues by disabling and reenabling the “Narcissus” addon however after another reload, they returned again.

I have unfortunately no clue what is causing this and I’d assume it’s not caused by Threatplates, so apologies for the time wasted!

[1] Whenever I hide my UI the TP nameplates are no longer hiding like before but instead stay visible (they even enable ones that shouldn’t be visible due to my occluded settings)

This is a bug in Musician (under certain conditions, when the UI is hidden, nameplates remain on screen even if the nameplates are disabled on Musician's side). This is not related to Threatplates and can be reproduced when Threatplates is disabled. This will be fixed in the next Musician release.

[2] When hiding my UI Threatplates just randomly shuffles the nameplates around and puts them on the wrong NPCs. In these two screenshots the only thing I did was hiding/enabling my UI.

I wasn't able to reproduce this issue on my side to figure out if it's caused by Musician or something else. If this happens again and you are able to reproduce, it would be nice to know exactly which addons you have, your Threatplates profile, CVars, WTF folder and so on. Eventually, try to reproduce with only Threatplates enabled and enable each other add-on one by one until you can reproduce again.