There is a bug in ReaSynth
when using extra sine waveform and portamento is >0, then tune of that extra-sine does not actually tune it but phase shifts/slides sideways and cut at period borders.
In screen capture and attached project file, there are two duplicates of the track with only difference that track 2 has Portamento set to "2" (ON), while on track 1 is set to "0" (OFF). I panned tracks hard L/R, put oscilloscope on each track and automated tune slider of extra sine so the behavior could be clearly seen.
example project file:
https://stash.reaper.fm/42058/202105...amento-bug.rpp
licecap gif capture:
videofile (with sound):
https://stash.reaper.fm/42062/202105...mento-bug.webm
audio mp3 render:
https://stash.reaper.fm/42061/202105...amento-bug.mp3
(Looking back to older releases, it's there since adding of Portamento options/parameter).
""" PLEASE""" It would be good to keep this behavior as it adds more sonic possibilities when modulating phases using this buggy techniqueb

So, please, fix but do not remove this incorrect behavior (maybe even develop it more in the future...)