Well sure, if you use a library in a borderline incomprehensible and clearly unintended manner and encounter an edge case then perhaps it isn't useful to report it. But while obscure, the usage you described (ie encountering stale config or cache data from a decommissioned install) can hardly be said to be unintended.

If an end user encounters an error while staying within the bounds of "expected usage" then the bug is guaranteed to be valid. You just don't necessarily know whose bug it actually is (for example a broken library versus broken usage of the library by the consuming program).