I tested tmux 2.3 and 2.6 which I have to hand, and the newer version seems to work well updating from an infinite command, but the old version does not. You seem to need at least 2.5, as you can see in the CHANGES file:
CHANGES FROM 2.4 TO 2.5, 09 May 2017
If a #() command doesn't exit, continue to read from it and use its last full line of output.