This sounds indeed unexpected. To narrow down the problem:
When does the Go app pick up the changed time zone? After some delay, or only after restarting the app?
Did you test this with another programming language, too? (To rule out that the issue is outside Go; for example, the shell might still deliver the old time zone until starting a new one.)
Do you have some minimal test code available so that others could try replicating it on their machines?
Which OS and shell are you using? Did you test with other OS’es and/or shells?