My guess is that neither FBReader nor Okular on the Desktop are native epub reading programs. They probably natively read some other format and convert an epub to it on the fly. This conversion probably does not understand basic css like display: none.
Versions of those programs for Android probably use the Android built in blink/chrome browser widget which natively understands css making epubs work properly.
But this is only a guess.
Update: based on this discussion
https://www.reddit.com/r/kde/comment...ular_and_epub/
I was right and okular's epub support is quite broken and uses obsolete conversion tools to try to read epubs.