SO2R signal attenuated on no focus TX slice

I use Flex 6700 + Mestro for SO2R operation.
There was a problem with SO2R operation, because the signal is attenuated by about 10-15dB on a slice where TX is not selected. The signal drop on a given slice is audible and visible on waterfall.

-Slice A has ANT A, Slice B has ANT B
-RF Gain = 0 on both
-Independent of software version (I checked)
-Independent of attached FDX
-Factory reset of 6700 performed
-Antennas directly to socket in 6700 (ANT 1 and ANT 2)

What to do so that the signal level does not decrease when I change TX focus.