alex.isakov said:

The issue was in the way.

I researched Lost Art of Oath of Faith and I selected 1 Imperial Seeker as attack unit in mining party (I received it as reward just before).

I sent the party before I finished 3rd level of the Lost Art.

I got a message about reward for level completion.

BUT then I got a message that I clashed with other mining party (it happens). However most strange, my attack party had 3 (THREE) but not 1 (ONE) Imperial Seeker!

How this was possible at all?

Greetings m'lord!

You might've been fooled by your keyboard, meaning that you accidentally pressed 3 instead of 1. 

However, if you're one hundred percent sure that you did not send those 3, please inform the Support Team!

Good luck!