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I think you can make a pretty good case for the spike being caused by the addition of the 100n and 10n capacitors: -

![Image alt text](https://electrical.codidact.com/uploads/eBqxkPb831i7BEGNn4ipJ9kr)

> *I assume that the solder blobs between capacitors introduced a series
> inductance that in turn caused the spike in impedance at 8MHz.*

It can't be ruled out but neither can the inductance of the yellow, orange and green wires on the test set-up. I mean, where do they go and what effect do they have I wonder?