I have measured length gains during rest periods (suggesting actual growth), but it was BPEL so that’s unreliable, it could have been an EQ fluctuation. I have also gotten reliable length gains (BPFSL) a day or two, maybe even faster, into PE after a long break, which is very fast (suggesting plastic deformation). Maybe both is happening.
There’s a simple way to test this for length with the help of the BPFSL measurement. If you measure BPFSL after every session you should be able to see what happens, also measure before a break and after it to see if you gained during the break. That would solve the mystery but it takes discipline to do something like that. It’s very important to use the BPFSL (and pressing the ruler against the pubic bone) becuase it’s the most reliable measurement for length IMO. It rules out EQ and fat pad fluctuation and also jamming the ruler beyond the pubic bone which could contaminate the measurement.