Thanks Beatriz - I really appreciate the offer of help. 🙏
I think what would be useful is for someone to let me know where to look to see if we have files missing. I have the same problem across all of our instances (local development, test, staging, and live), so I can't compare what might be different from a "working" instance.
We have a git upgrade and deployment process (similar to this) so perhaps some files have not downloaded or updated properly?
I updated to 3.9.2+ (Build: 20201002) last night, but that didn't help.
This forum thread on the H5P site is very similar to the problem we face, but the suggested solution didn't help and it's a pretty old thread anyway, so perhaps not relevant in terms of a solution.