I don't know if it is reproducible but playing as the US I took control of my ally Canada's military. Spain had declared war on the allies and proceded to take over much of France. The Germany surrenders event had fired and Europe was freed so France was once again in control of France proper. I landed several Canadian division near Brest and my US division were rolling up the Spaniards from the SOuth from my still controlled Italian provinces. The canadian division stayed out of supply until the province they were in flipped to French territory. I don't know when that happened as I was busy with my units. When they were then in supply I proceeded to use them to now take over Spanish held French territory. When they did so the territory went to Canadian control.
When our forces met I began to use them in conjunction with my forces when the "enemy in sight" boxes showed up. When they entered French territory controlled by the US the messages would appear. And keep on appearing every hour on the clock until the unit was no longer in that territory. The same thing happened when my units transited Canadian territory in France. Control of the territory would also flip based on the last unit through.
When our forces met I began to use them in conjunction with my forces when the "enemy in sight" boxes showed up. When they entered French territory controlled by the US the messages would appear. And keep on appearing every hour on the clock until the unit was no longer in that territory. The same thing happened when my units transited Canadian territory in France. Control of the territory would also flip based on the last unit through.