Timed_commands broken (gr-uhd / 3.7.1)?

Hello list,

after a lot of working, I finally came around to write a minimal example
to show what does not work.
To make it short: from python, calling
src = uhd.usrp_source(…)
[…]
src.set_command_time(uhd.time_spec())
src.set_center_freq(1e9)
src.clear_command_time()
does not issue the tuning request a second in the future; it is
executed right away.
However, the next get_time_now() call happens a second delayed.

The complete test can be found under
Testcase for timed commands. That fails. · GitHub or cloned via
git clone Testcase for timed commands. That fails. · GitHub

Could please someone verify this problem is specific to my system?

Thanks in advance,
Marcus