30 EIRP for 5.1 GHz

On most of our 450m 5 GHz AP's we can go up to 36 EIRP in 5.1 GHz. we have one unit that doesn't have the mu-mimo license that is limited to 30 EIRP in that band. is that a limitation of MU-MIMO functionality?

This is in the U.S.

No, it doesn't have anything to do with MU-MIMO functionality. Rather, it is more likely that you're closer to the band edge. 

In some cases, the EIRP must be turned down to lessen the "out of band" emissions.