The role backs up and snapshots everything without checking whether the user provided version is a valid version upgrade OR whether it is available. It does check - however - whether the user provided input is valid from a semantics perspective (only) within upgrade/main.yml where it is able to catch most of user mistakes.

Therefore I'd consider this issue as an enhancement, rather than a bug per se.

I will submit a commit to address some aspects of the upgrade logic (for example, the fact that an upgrade to latest doesn't work), and include a check to determine whether there are upgrades available, which will fit nicely when one specifies latest but already is under whatever latest is. ;-)