Hey @herr rilke thx for sending me the files.
The good news is that you can please mark this topic as [solved] here in the RockPageBuilder forum, because it is not related to RPB but rather to RockFrontend.
The bad news is that the way I designed RockFrontends assets feature does not reliably work with template cache at all. It works with no cache, it works with procache, but it doesn't work with template cache.
The problem is that with ProCache we get a 1:1 copy of the rendered page without calling any php files on the request. That's fine, because then all assets are already injected in the <head> and the browser just loads the static files.
If template cache is active it's different: Files like _init.php or layout.latte are not loaded and also not all the template php files, but all other php files like RockFrontend.module.php or RockPageBuilder.module.php are loaded. So it's a problem to place $rockfrontend->styles()->add(...) in files that are not loaded and other calls of styles()->add() in files that are. That's what you are seeing. RockPageBuilder assets are being loaded from RockPageBuilder.module.php but uikit files are not, because they are injected from _init.php or layout.latte or _main.php (wherever one might place it) which is skipped by template cache.
To be honest I'm not yet sure what to do about this as this would mean a major rewrite of all my projects and likely any project built on top of RockFrontend.
The easiest fix for your situation would be to just use ProCache, which is worth every cent.
If anybody has a good idea please let me know. I'll have to think about a good solution...