Both plans are configured as exclusive. Both plans do not have the restriction of upgrading only if it is more expensive.
The puzzling thing is that there are two members with the same active subscription, and it displays for one but not the other as an upgrade option (both are visible in backend only).
The only difference I can see between the users is that the one where it is not displayed has either an expired or unsubscribed subscription for the plan which is not displaying.
If I get rid of that old subscription in MySQL, it then works.