I forgot to change the version number in v0.81. Don't worry if you still see v0.80. Since the code has not changed, the version number doesn't matter much, as long as you see v0.80 or v0.81 AND Help -> Mkvmerge version shows version 8.6.1. However, if you use Help -> Check for update, it will tell you that v0.81 is available.
I have uploaded a new v0.81 with the correct version number. You can download it if you wish:
BD3D2MK3D.7z