Bit confusing so I'll try to explain:
1. I have inherited a few next-gen projects with many issues; some in epics, some not.
2. I'm creating a scrum board that includes issues from several projects
3. The scrum board and its filter are in a new classic project I created just for this purpose
4. The filter shows all the issues I want in my backlog
5. The scrum board omits any issues from next-gen projects that have an epic but shows all the other issues.
6. Yes, I've properly mapped all the columns.
Is this a bug?
Here are the issues in my filter:

Here is the backlog. As you can see it omits the issue from a next-gen project that has an epic but includes all the other issues:

EDITED TO CLARIFY:
The only way I can combine issues from multiple projects into one scrum board is by putting the board and its filter in a classic project with a custom filter.
The filter is working fine, but the backlog based on it has missing issues as described above.