UBNT PowerBeam M5 (PBEM5) vs UBNT Nanobridge (NBM5) perfomance

Has anyone noticed a fairly extreme difference on "on-air" performance between UBNT NanoBridge (NBM5 XM) and PowerBeam (PBEM5-M400 XW) units? I noticed long ago when I replaced our tower-mounted NBM5 units with the "new and improved" Powerbeams that we took a "hit" in system throughput. Local testing between units at my home location yield a difference of Internet throughput 39.56D/23.77U (Mbps) using NBM5 endpoint as compared to 9.58D/9.31U (Mbps) using PBEM5 endpoint. Currently running Nightly 613, but this has pretty much been the case since we upgraded to PBEM5 from NBM5 on our tower 1/3/21 using Nightly version 38.  Pretty much same variances when comparing IperfSpeed results.  Could it be possible that the PBEM5 isn't truly running "MIMO"?