The problem is still there since the external player will not honor the "default track" flag in MKVs.You might try this:
There is an option inside ZDMC to use the Realtek SoC for playback. ZDMC > System > Video > Playback > Turn on "Play video with external player". Inside ZDMC the Realtek SoC is referred to as the external player. I'm not sure how this combination might work with forced subtitles with MKV's but you might give it a try.
I use primarily ISO's with Home Theater 2.0 and forced sub's are supported.
You are not seeing the problem since ISOs will playback using mpls instead of direct file.