You have 2 process definitions deployed with multiple versions each.
You are on the instance migration view.
Change the target process definition to the second process.
Change back to the original process definition.
the version is not the latest version of the original process but the version of the second selected process.
That when switching definitions it always chooses the latest version of the selected process
This can be confusing, lead to unintended instance migrations.
The same unexpected behaviour can be observed when changing the 'source' process definition.