Last week I did some more diagnostics and I printed an M12 probe holder to try that.
When switching the probe I noticed the sovol probe made a rattling noise. I opened it up and found that the ferrite core around the spools has snapped in half. This is probably caused by the nozzle crash I had earlier.
This explains the varying reading as the resonance of the coil was changing with the loose piece of ferrite moving around in the casing.
With the M12 probe the problem disappeared.
I got in contact with sovol and they will send me a replacement.