I leave Truepianos Reverb set to Off, and it stays there. If it's changing to some level other than what is set in the TP GUI when playback starts, there can be three causes:
1. The Reverb control widget in the MIDI track is active; right-click the "Rev" knob in the MIDI Track Inspector, and choose Disable Control if it's not already checked. Moving it will re-enable it automatically and alter the TP setting. Enabled track control settings are re-sent every time playback is started.
2. There are CC91 Reverb Level messages in the MIDI clip.
3. There's a MIDI automation envelope on the track controlling Reverb (works by sending CC91