Could it be that they are part of a Field marshals army group and that Fieldmarshal has an actual front line for said army group ? If you then take an army and put it soemwhere else, it automatically rejoins the field marshals line when its own frontline is removed.
The issue was/is that troops are assigning themselves to a new front line, without completing the orders associated with the previous front line. In that case it was a combination of the known "front line disappears bug" and the fact that troops will find a new frontline automatically...overseas.
I would appreciate if the controls in this game improved. Instead they have regressed, and some of that regression is by (arbitrary and unjustifiable) design.