Right so one of the 'features' of a high ranked vampire is they can use their powers and feed without changing into their Dark Form. I guess it is to simulate they are now so all-powerful that they do not need to even harness the 'dark' to do vampy things.
Deleting the localthumbcache.packge file from your TheSims 4 folder after every time you play when using mods is a good idea. Deleting it every now and then when you play without mods is all that is needed.
To figure out which mod is causing the issue and remember it may still be a bug but just one that I can't recreate, first take out all mods, delete the localthumbcache.packge file from your TheSims 4 folder. Start a new game. Play the now vanilla mod-less and cc-less game and test to see if the bug is still there. If the bug is still there then you know it is a code issue that will need the TS4 developers to fix (unless you make mods or can fix code yourself.) So at that point you can just simply throw all your mods back in and keep playing. Making a Bug Report is one way to bring the bug to the developers attention.
If you do not see the bug you can then place half your mods back in - test to see if the bug still occurs. If it does then you know that one of the mods you just replaced into your game is causing the issue. So quit and delete the localthumbcache.packge file from your TheSims 4 folder again and remove half the mods again.
Keep doing this remove half your mods testing pattern until you can narrow it down to just one mod and only one mod in your game when the bug occurs. Then contact the person who made the mod and explain the issue. Hopefully you can help them to narrow down what is causing the issue with their mod by telling them exactly what you did in-game to see it.
How to Fix Mod IssuesHow to Make a Bug Report
https://www.youtube.com/watch?v=eLW2cYKpOl8Link Directly to EA's Official Bug Forum