Encoder-decoder mismatch in ECM-3.1 when inter MTS is enabled in the case of CTU size is equal to 256.

In ECM-3.1, encoder-decoder mismatch happens when inter MTS is enabled in the case of CTU size is equal to 256.

For example:

EncoderAppStatic -c ./cfg/encoder_randomaccess_ecm.cfg -c ./cfg/per-class/classC_randomaccess.cfg -c ./cfg/per-sequence/BasketballDrill.cfg -i BasketballDrill.yuv --SourceWidth=832 --SourceHeight=480 --FrameRate=60 --InputBitDepth=8 -q 47 --FramesToBeEncoded=5 --IntraPeriod=64 -v 6 -dph 1 --MTS=3 --CTUSize=256

The mismatch also happens in classA.

It seems because mtsIdx is searched in each TU split from 256x256 CU but the mtsIdx is only coded in CU level.