You can see in the gif:
1. Playback starts with track 2 soloed.
2. Toggle solo to track 1 does NOT cause note off on track 3
3. Toggle solo to track 2 from track 1 DOES cause note off on track 3
-- Beginning the process on track 1 will result in note off
-- on first use of
Track: Toggle solo for selected tracks
4. Subsequent use of the action does not cause note offs.
-- Playback stopped at 0:08 causes last note off in log.
Solo in front is on in the gif, but I don't think it affects anything related to the issue.
Steps to reproduce: Set up a new project that looks like the one in the gif. Use long unbroken note in MIDI item. Put ReaControlMIDI on track 3.
REAPER 5.32x64 Windows 7