Good evening, I had updates disabled because I have a mid-2015 iMac that doesn’t support Mac OS Ventura (it would need to be patched, and I don’t like doing that). The 99.0 update has appeared automatically, even though I can’t install it. The latest official version of my operating system is Mac OS Monterey 12.7.6, and versions 90 through 99 are incompatible and I can’t install them. The last version compatible with my system is 89.0, so it shouldn’t be detected or activated automatically.
The update check is a generic one that doesn’t take availability on your current OS into account. There is a new release available, therefore the dialog pops up once for each new release made.
You can disable the check in the preferences if it annoys you.
Note that MKVToolNix does not contain an automatic update mechanism. All it does is show the dialog. You have to perform the update yourself if you want. There’s no “automatic activation”.