When you edit the first sample of the sample, when its looping, then renoise adds somehow high frequencies to it, till the sample loop restarts. This only happens, when the interpolation algorithm wasn’t set to none.
Thanks for the detailed report. This is indeed a little bit weird, but expected.
Renoise adjusts its internal representation of the sample data at the loop boundaries for the resampler, in order to improve the precision of the interpolation depending on the current loop mode and loop marker positions.
It does so only when a note starts to play, so you may hear some slight differences during playback when manually modifying the sample.
Your example is pretty extreme. In most situations this will hardly be audible and always fixes itself when playing the sample again.