Since you have a Windows PC and a MacBook you can also use the free software Room Equalisation Wizard (REW) to check the results you get. The UMIK-2 is one of the recommended choices for use with REW.
In fact, you can even use it for a much more fine grained room correction and many other things. It just takes some learning to get started.
We still don't have enough data points for a valid statistical analysis, but
@Wiimer might have had the right idea here. The confirmed troublesome cases all appear to be related to Meditec SoCs.
I am very sure that this sharp 10 dB step you get around 55 Hz is
not real but an artefact.