Ran coops on three Holding Actions. In the first 2, we had "host left" (no biggie, as we know very common, usually a user problem not a server one).
However after first mission we both backed out of the server and then back in. After the second host left, we both rebooted and then came back in.
On the third, rather unusual, neither of us saw a host left message, and neither of us saw a Mission Complete (or incomplete fyi) ... the game simply closed (as normal). No PP awarded on any of them (for me at least).
Like I said ... very good chance this is a user issue and not a server one ... but wanted to cover the bases.