i think downsample ac3 to stereo may be enabled as default setting in MP Config under MPA decoder in expert mode i think.
I checked, and indeed in the MPA decoder settings the option "downmix AAC to stereo" was checked. But unchecking it did not do anything to the TV sound. And theoretically it should not have any effect, because I am using AC3filter for AC3, and the Monogram filter for LATM.
Anyway, adding AC3filter to TV Postprocessing seems to do the job for me, even when the audio is LATM.