@rob any way you can use a heroku instance with an older version of Node / npm?
Wanted to post earlier but had to wait for the post to be accepted, but you are correct. In case anyone else has the issue, this was it:
In package.json the 'engine' line for node was =>10 so Heroku was choosing Node 15. Manually changed that value to the same as what works for me locally 12.14.1 - all works well. Now having fun setting up nodebb-plugin-session-sharing - cheers!!