This seems to be potentially ambiguous:
tell me [something] when you get back OR tell me the time you (will) get back

And I quote someone else,
"Please tell me when you get back. = He will get back before he tells you OR He will tell you his planned return time.
Please tell me when you'll get back. = He will tell you his planned return time.
etc."
Why is this then not ambiguous?

>Why is this then not ambiguous?

The above sentence isn't ambiguous. It means "When I come back, I'll tell you something." It doesn't and cannot mean that the person is indicating the time that he/she will return. That would be "I'll tell you when I'll be back." or similar.

> "Please tell me when you get back. = He will get back before he tells you
- correct, I'm asking him to tell me something after he gets back.

>OR He will tell you his planned return time.
- no, "Please tell me when you get back" can't mean that. - see sentence below for that meaning.

Please tell me when you'll get back. = He will tell you his planned return time. - correct

Well, I guess that was a minority opinion... there are too many varieties to get a hang of everything :$