I was able to create a similar situation to my previous post using v1.16b2.
Please note that this is a test case to demonstrate MTSG and not from
an actual game
Picture above shows
McClellan's Army in Culpeper with Porter's and Franklin's Corps, total 34152 men, 9788 horses, 156 guns
Fremont's Army in Manassas with Heintzelman's, Sumner's and Keyes' Corps, total 45231 men, 12392 horses and 252 guns
Banks' Army in Spotsylvania with McDowell's Corps, total 21611 men, 5278 horses and 96 guns
Total USA forces are 100994 men, 27458 horses and 504 guns
Neither Fremont nor Banks should be eligible for MTSG into Culpeper (different army)
Additionally Fremont, Heintzelman, Sumner, and McDowell are all inactive (brown envelopes)
Banks' Army and McDowell's Corps are ordered to move by rail to Albemarle, synchronised movement on, and will arrive on day 2
Picture above shows the CSA move for the same turn.
Longstreet is ordered to attack McClellan in Culpeper, and will arrive on day 9. Longstreet has 31166 men, 4594 horses and 149 guns
Picture above shows the battle results
The CSA forces are Longstreet's 31166 men as expected.
The USA forces are 100904 men, 27428 horses and 504 guns which closely matches the total US forces for all three armies as calculated previously.
[The slight discrepancy in numbers of men and horses is presumably due to the game engine's calculation methods]
You can also see that all the 3 star generals were at the battle. There are also three army HQs shown as present.
It would appear that every single US army and corps has marched to the sound of the guns even though none were eligible to do so.
Save game attached below.