I spotted something interesting the other day when accepting my weekly Flashpoint award (this is the three-flashpoints per week one); I had my Planetary commendations at 100/100 and my Ultimate commendations at 287/300 and I forget what my Elite commendations were (~150?).

Now, considering that the weekly reward for completing 3 HM FPs is 10 Planetary, 10 Elite and 12 Ultimate commendations, one would expect that if one of the three categories is full, one simply would not get any of that commendation type. However, I was viewing my currency tab at the time and my Ultimate commendation could did not go up to 299 as I'd expected, it remained at 287. I don't remember the original number, but I suspect Elite commendations suffered similarly. I have since spent some Planetary commendations and can now accept Ultimate and Elite commendations correctly from the weekly rewards.