Is bug with ps about code logic checker?

IDE version:  2017.3.1 to 2018.3.3

If I use "or" between (!isset($a)) and (!isset($b)) , IDE call "$b might have not been defined", but it should be return before var_dump($b).

IDE should not call the tip.

But while use "and", IDE not call any notice about $b. Should be notice, in this case, $b might not be defined. So confuse me.