Function calls not highlighted in f-strings within {}

[nst0022]

Line 1 shows a user-defined function.
In line 4, the call to hello() is properly highlighted.
Line 6 shows the member function join(), which is also properly highlighted.

However, both functions are not highlighted within f-strings (line 8 and 10).

I could not figure out from python.vim, why the build-in function str(), and the reserved words 'for' and 'in' are highlighted, but the above mentioned functions not.

Is this a bug?

[nst0022]

it works now