I notice that when I recruit a character into one of my open slots, that I am not receiving XP for the recruitment. I have added some additional cover slots to duplicate some of the 1* and 2* characters in my roster. However, if I add another cover to the character I just recruited, then I do receive the XP for training that character. I am not completely certain, but I think when I added the new 4* Hulk after the Hearts of Darkness PVE, I also did not receive XP for that recruitment.
I reported this as a support issue (Ticket #365880) but the support person assumed that my issue was XP awards for completing story events.