I can confirm this bug. I am having the same issue with Veles.
Veles DO send to the acropolis.
When I click "Recall" to get them from the acropolis, they do NOT show up.
When I click to Recall ALL my defense from the acropolis, they do NOT come with my other defensive troops to my castle.
When I look inside my acropolis, they DO show in the list.
When I send reinforcements, they do NOT show in the menu, nor would they send if I sent all my defense as reinforcements to a castle.
When I attempt to do positions, they do NOT show up in my acropolis list of troops, but they DO show up in my castle list of troops. In other words, I can "send" them from the castle reserves, but not the acropolis. HOWEVER - even when I send them from the castle, they do NOT actually go to the position. They will stay behind in the castle. I've not tried sending only Veles to a position out of fear of something weird happening or me just losing the Veles.
If you need more information, do not hesitate to ask. Veles are clearly broken right now, at least for me.
I've actually narrowed it down a little bit more. I decided to do a bit of research and I've discovered that the Veles are negatively interacting with another troop.
My methodology: When I click on a menu to see all my troops, I don't see the Veles when all my other defensive troops are showing. However, I've noticed that I see them when they are being produced and only a few other troops are being produced. This leads me to two thoughts:
A. Either I have too many troops and it's neglecting to show me the Veles (unlikely), or
B. Another troop is coded similarly and therefore it's not showing me the Veles (more likely).
So I've sent troops one type at a time until I can see the Veles.
What I've discovered? Hypaspists. When they are together with the Veles, I have no control over the Veles. In other words: when the Hypaspists and the Veles are together in my castle and I click on "Send to acropolis", the Veles do not show up in my defensive troop list. I'm assuming this will be true with the other spots where the Veles fail to show up.
To me, this means the problem is an interaction between the Hypaspists and the Veles.
I cannot speak for the Tririum (spelling?) troops that the OP spoke of, as I do not have any of that troop type.
This also isn't exhaustive. I had ALL my defensive troops in the acropolis and decided to test the relationship exclusively. So I brought out the Hypaspists by themselves, waited the 15s, then went to bring out the Veles. They weren't showing. So I brought out the Standard-Bearers and waited 15s. Now the Veles are showing in my Acropolis and I can bring them out.
So I believe it must be more than just Hypaspists ... and must be with Champion units in general, somehow? I'm not sure.
And now when I have just those 3 types (Hypaspists, Standard-Bearers and Veles) in my castle and click to send them to the Acropolis, all 3 show up.
I give up. My experiment is over and I'm not 100% sure what the heck is causing this weird interaction. I feel as though I've now debunked my theory on the Hypaspists. Good grief.
Dragonian, to see that I could also confirm a general bug, or if plarium had new update of invisible troops... I tried again with more units...grabbed StandardBearer & hypasphist from acrop with triarus & veles, once in city sent them out as reins, brought back to city, then back to acrop.