as Tom J Nowell asked in the comments a caching plugin was the culprit. In this case WP Fastest cache. Just deleting the cache did not work. Turning the caching off and on again fixed the problem.
update: I did some more digging and the fact it gets commented is correct. The caching plugin also runs a code minimizer. For some reason the minimized code did not use the stylesheet I added. Turning it off and on again forced a rerun of the minimizer (I guess).