Hi,
This reminds me a similar issue I was having with my Mini v2.2: LimeSDR Mini V2 reading samples from RX FIFO - #2 by Zack
Not sure there is something you can do from just user side of gnuradio, I am not a huge expert of it. Maybe there is a way to configure TX channel without starting a stream.
Perhaps gnuradio can adopt some sort of quick fix, similar to the examples in the LimeSuite: Fix singleRX example to work correctly with LimeSDR Mini V2 · myriadrf/LimeSuite@e353507 · GitHub (at least until LimeSuiteNG is stable as it provides much cleaner API for developers, and also supports new XTRX hardware).