Thanks for the example files. Upon further testing with several files and settings, I found out something interesting.
It seems that if Reaper is set up to import existing MIDI files as .MID file reference, the files can not be opened in inline MIDI editor if they are of Type 0 MIDI files. IOW, only MIDI files which are of Type 1 (multitrack), can be opened and edited in inline editor with that condition.
I went back to Reaper v5.0 with the same results, so if it has ever worked, it would've been even earlier. I think this requires a bug report, or if it never was a feature, feature request at least.
edit: I filed a bug report about the issue;
https://forum.cockos.com/showthread....66#post2200966