Hello, I found another similar post from january which was archived for reasons unknown to me, as the bug still exists within the game. (Edit: just noticed its more than a year old)
As shown in the images here: https://imgur.com/a/cbqLpl3 you can see that I took screenshots of the current XP of my warframes before and after running the same mission. You can see the kills increasing for both, but the XP only updates on Equinox and i have no clue why. As in the post I linked earlier i also tried formaing saryn, tried a bunch of different missions to no avail. Not game breaking but really annoys me for some reasons.