In OneDev an issue can be associated with multiple milestones, and the backlog by default only shows issues not associated with current milestone, instead of any milestone. This is because:

1. There might exist different milestone scopes, for instance, release milestone and sprint milestone. An issue scheduled into release milestone but not into any sprint milestone should be considered a backlog issue when planning sprints.
2. An issue might exist in a milestone, but postponed to next milestone for some reason. The history milestones an issue belongs to should also be kept to make milestone statistics (burndown chart) to be still meaningful.

So backlog issue is something very specific to site, and OneDev only puts minimum constraint on them (only filters those not belong to current milestone) . If you do not care about above points, you may simply edit your board to specify backlog base query as

"Milestone" is empty

Back log description needs to be updated to reflect this though

If you do not care about above points, you may simply edit your board to specify backlog base query as

"Milestone" is empty

I could swear I had done this last week and got an empty result, thinking that onedev produced a query like milestone is empty and milestone is <selected milestone in ui>. But it seems to work now. So if you find "Milestone" in the backlog query you do not apply the ui selection in the background?

Then, as you said, UI help text should be updated to reflect the current behavior. Maybe also making clear that you can override the UI selection by added "Milestone" filters to the backlog query.

If an issue is in backlog, OneDev will and sub query not("Milestone" is "current milestone") with the base query.

Closing it now, reopen if you observe odd things.

@robin you really thought of almost anything didn't you! Love this! :)

@jorydekort thanks for the kind words, and all the good suggestions. 😃