After temporary assignment to an environment variable, that variable should be unset after the function exit, provided it was unset before. Consider this (using the new tmp syntax):

⬥ date; has-env TZ; { tmp E:TZ = UTC; date }; date; has-env TZ; put $E:TZ
Sat Dec 11 10:07:40 CET 2021
⮕ $false
Sat Dec 11 09:07:40 UTC 2021
Sat Dec 11 09:07:40 UTC 2021
⮕ $true
⮕ ''

As can be seen from the various date outputs, it does make a difference whether an environment variable is unset or merely empty.