IDX/SUB offset ignored when muxed

[ZetaStax]

I have a IDX file that contains the following:

size: 720x576
org: 110, 0
scale: 70%, 100%

When I play the video with MPC-HC with the subtitle as an external file, the placement is exactly as I want it. However, when I mux the subtitle in the video (MKVMerge), the offset is ignored and the subtitle is placed at (0,0). If I demux the subtitle I can confirm that the IDX file was not edited. I could not find any option with MKVMerge that would pertain to this behavior, so what gives?

Unfortunately I have to make this work, the video is anamorphic 16:9 and without the downscaling the text is stretched very unpleasantly. I'd rather not have to OCR the subtitle either as this process requires a lot of proof-reading...

[El Heggunte]

MAYBE the problem is in LAV Splitter or/and in the Internal Subtitle Renderer of MPC-HC...

does the problem still happen if/when you use VSFilter

P.S.: Also,

the video is anamorphic 16:9

Have you set the correct aspect ratio in MKVmerge's GUI

OR configured LAV Splitter /Video_Decoder "properly"

[ZetaStax]

Hmm you're right the problem is not exhibited with a different subtitle renderer (tried to play the video in Kodi). It doesn't look like I can get it to work with the Internal Renderer of MPC-HC, which is gonna be a problem in my case. Any idea? (short of converting the IDX/SUB to SUP perhaps)