I don’t think I agree with your interpretation of this not being a bug and slowing down the algorithm.
From my point of view, any package manager should only downgrade a version you already have installed if a newly added package requires an older version for compatibility. In this case, it is downgraded without such a reason. We could assume that a restriction exists and the bug is that it is not displayed in the REPL. If that was the case, then manually updating the downgraded package would be stopped, which doesn’t happen. The other case is that there’s a bug in the that step too, and it fails to detect the restriction at that step and allows the user to update the package.
Also, the microseconds it takes to check version compatibility in toml files vs the time it takes to precompile a bunch of files all over again everytime this bug occurs are such that I think a “heavy but safer” compatibility algorithm is worth it.
But we’re discussing hypothetically, since I myself would like someone else to help us confirm if this is a bug or not.