Strange, believe the status should be "Upgraded to other". Please perform a test purchase using the dummy test gateway then immediately upgrade using the dummy test gateway and confirm what status the old subscription as showing. This should help confirm something isn't working quite right.

You'd need to configure an upgrade condition under the Workflows tab to do that, but you could also turn on Prorating under Pricing and it'd prorate their remaining value towards their upgrade.

I thought I had already done the proper workflow to only allow upgrades during bonus months and grace period.

The plan should only be visible then if they have one of the selected plans in any state at any point in time. So if they've ever had a subscription to one of those plans ever then it'll allow them to upgrade. Create a test user that has never had a subscription and see if they can upgrade to that plan.

Still doesn't answer why "upgraded to other" suddenly became "expired".

If new subscriptions are working fine (as they appear to with your screenshot from earlier) it could've been a bug in older release that's now fixed.