When using the UF1/UF8 with Reaper, you may find that when engaging the Play or Stop button, the faders revert to -11.1db. This issue is only present on Mac.
To prevent this from happening, please make sure that you do not have your Midi Output ports enabled in the Audio/Midi Outputs tab in Reapers preferences.
Pitch Bend/Wheel messages are used for UF8/UF1 fader positions. It seems Reaper is sending out a Pitch Bend reset to all enabled MIDI outputs on start and stop, presumably along with other reset messages, and this value is reflected as 11.1-db.