H5P was relying on an implementation detail of core's JavaScript and CSS aggregation - that it would write out files when generating a URL. This no longer happens when the URL is generated but in a system module asset controller.
In 🐛 Drupal 10 compatibility JS aggregation causes js error with aggregation enabled RTBC services using the old, deprecated classes from Drupal 9/10.0 were added to maintain the same as the current behaviour. Instead, the logic should be changed either generate the URL and use the route if possible, or if not to copy some logic from the controller to write it out instead.
In general I wonder if instead of 'faking' libraries, whether dynamic libraries could be created instead, and then the whole logic could use #attached instead of re-implementing parts of Drupal's asset handling.
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Since this can be postponed until Drupal 11 support, marking it as Normal instead of Critical.
Going ahead here to unblock the 11.x compatibility issue.
Automatically closed - issue fixed for 2 weeks with no activity.