I am assuming that this is related to the way that CSS is generated from dynamic style sheets in the Boost theme. When I switch to a new theme and purge caches, it works again. I can also turn on theme developer mode and then it will work with Boost.
After the CSS failed to load initially, I also found that pages weren't loading at all (timing out after 300 seconds) so had to purge caches via CLI.
Has anyone else had this issue?
I guess there muse be some styling in H5P that doesn't parse correctly.