No idea how to reproduce it. Entire army locks onto it, is slowly killed by it.
I was experimenting with this too and I think the invincibility happens when they get killed by a defensive building such as a Town Center or a blockhouse like you see here in the screenshot. Instead of dying they get negative 2 billion hp and can’t be killed from that point onward.
According to the changelog they changed some animations around so that change might be what’s causing it. They also go invisible to the player when they enter melee mode but you can still select them if you have assigned them to a command group and move them. Enemies can see them too. Very Bizarre Bug.
I’ve seen the same patch note on Eagle Runner Knights but idk if the bug applies to them.
I know the Aztecs could have used a slight buff but idk about adding immortal units to their roster lol. The Mexicans certainly don’t need it.
It was also going invisible for us, you’re right, it was going in and out of melee mode because it was reacting to the proximity of cavalry. Didn’t realise what I was seeing, but can confirm that too.
What a bug funny af lol
It looks like the new eagle runner animation set (also used by the holcan javelins) are missing a melee run animation or something, which causes them to go invisible. I’m guessing this is probably causing all kinds of spaghetti code mess 
Hi! Thanks, we are already tracking this!