Thank you for reporting this.
I just looked into this. I performed the OSUpdate on some iOS devices via SM and then looked at their raw MDM responses before and after. What's interesting about this Rapid Security Response OSUpdate: the BuildVersion and OSVersion reported by the device via MDM requests appears the same before and after this update. After my device was on 16.4.1(a) locally it still reported OS information via MDM commands like this:
"BuildVersion"=>"20E252"
"OSVersion"=>"16.4.1"
SM periodically sends MDM commands for this information to keep its information updated in Dashboard, but it looks like the devices have identical BuildVersion and OSVersion reported via MDM after this specific 16.4.1(a) Rapid Security Response update. I can see 16.4.1(a) locally on the device in Settings, but via the MDM command its raw response is still just "16.4.1" (which is why SM shows the value like this in Meraki Dashboard).
Since this is the first Rapid Security Response OSUpdate on iOS there may still be some kinks to work out-- we will be sharing feedback with Apple to help improve this for future updates. Thank you for bringing this up!