phpstan: Incorrect variable always exists

Summary of a problem or a feature request

PHPStan incorrectly reports that a variable always exists in some cases.

Code snippet that reproduces the problem

https://phpstan.org/r/b0e37609338d0c49645295f039a0e73d

Expected output

PHPStan complains that Variable $handle in isset() always exists and is not nullable.. However, https://3v4l.org/nUfFW shows the actual output is ko, which means the isset gets evaluated to false.