Hi,
I had the same problem.
I use a 12V input to power the BiasT for the LNB, the preamplifier/PA stage, a fan, and the Pluto+ SDR with voltage converters. This setup has plenty of room for EMI/EMC issues.
Initially, I had a cheap LSP-02G LNB, which sometimes kept losing the beacon stabilization during transmission, although RX worked visually fine during TX.
I upgraded to a Bullseye LNB to fix the loss of beacon stabilization issue, but then I experienced only noise in the SDR console during TX. With a separate power supply for the LNB, both RX and TX worked perfectly.
I believe the Bullseye LNB is more sensitive to power supply interference than the LSP-02G. Although this interference likely caused the beacon stabilization issues with the LSP-02G as well.
TLDR
I added a step-up converter before the BiasT for the LNB, and all problems disappeared. The step-up converter probably filters the interference at the input, preventing it from affecting the output.