I don’t know if it is known 
If armies are grouped together like this:
It is imposible to merge single armies:
If armies are in middle of traveling together and I want to drop down (ungroup) them for merging. Sometimes all except main army are pulled back to point that they are travellin, but a moment before they all were in middle of the way between points. After refreshing everything is ok.
Before:
Action:
After:
It is more probable to happen if there are a lot of armies grouped together.
If I click “ungroup” then “merge” right after, sometimes something like this happens:
and after cancel I cannot select regrouped unit:
And I cannot group it back. I have to refresh game.
If I want to merge units in the middle of the road target is always displayed on the node instead of road:
So this is quite confusing.