I have experienced this. Latest release as of this date (1. 8?)
All civilian garrisons have disappeared, my Lunar, Martian and Europa garrisons are all gone. Every single unit I had on Earth remains, however they have lost their OOB and are all completely detached now.
Normal game, nothing weird. Saved a few days ago and took a break. Came back and only noticed because I was suddenly getting events about unrest on colonies with no military presence.
Only thing I can think is that, at one point, I did hit the "Clear hierarchy" button when I had made a mess of the OOB but I never checked what the button does.
Additionally, I had a handful of Marine companies in boarding capable transport bays on ships I had sent to battle. Ships were decimated but I got no Event for the destruction of the ground unit aboard. Unsure if that's intentional.
EDIT:
Also, just checked previous saves (the automatic ones in the root folder) and the problem exists at the earliest point but that was only 20 mins before the most recent save.