Stealth units are supposed to ambush armies that step onto their tile. That much is expected behaviour.  However, yes, there is definitely currently concern/frustration that stealth units also ambush the armies of allies, client states, all envoys, etc., that we almost always don't want them to ambush.

In the meantime, to make the most of an ambush when you want to, put your combat-stealth units in choke points and surprise hostile armies advancing on your capital, or keep them near other armies as surprise reinforcements.  Prowlers are a bit sub-par for their era, but Partisans and emblematic stealth units from the Haudenosaunee, Cubans, and others can be dangerous.

Firs of all, THANK YOU for the explanation and opening to me the concept of spies and in general of hidden armies. I will make a better use of stealth mechanics from now on.

Regarding the concern in bold in your post, which I share, may I suggest to add a setting (on the hidden unit or in general) for the type of units to be automatically targeted by hidden armies?

Have a good day

Matteo  

Hello!

Thank you very much for your report :)

The issue is being investigated. 

Have a lovely day! ^^

I can well imagine that it will not be easy to find a reasonable solution. The question is, can the foreign unit enter the field with our hidden unit? If yes, then 2 units are standing on one field, which is probably not possible from a game technical point of view. Or the foreign unit will not be able to enter the field, like a mountain field. If the foreign unit is in multiplayer game, then my opponent automatically knows where my hidden units are. This is certainly not intentional. 

The first solution with two units on one field is better in my opinion, but requires strong change of the game code. Oh ... it is not simple Amplitude. 

But there is a third solution, it doesn't require any change, it just needs to be explained why our allied friends can be killed from time to time. A plausible explanation is enough ;) 

That was a joke ... sorry :D 

There should be a toggle for auto-ambush, just like there is for auto-expore.

If unit finishes it's movement on tile with hidden unit, hidden unit should be force-moved to nearest free tile in direction opposite of other unit advancement. Or revealed, if cornered with no means to free tile for advancing unit.