Hey, this is a reaper generic question, not really linked to reapack - but here's what you can do. Close reaper, edit reaper-kb.ini with a text editor in the resources folder (Options > Show Reaper resource path in explorer/finder), find the line :

SCR 516 0 RSdbc8f7339a911c9fb3b7a75b94a672d396b75b39 "Custom: cfillion_Clamp velocity of selected MIDI notes.lua" "ReaTeam Scripts/MIDI Editor/cfillion_Clamp velocity of selected MIDI notes.lua"

And replace the flags 516 by 4 :

SCR 4 0 RSdbc8f7339a911c9fb3b7a75b94a672d396b75b39 "Custom: cfillion_Clamp velocity of selected MIDI notes.lua" "ReaTeam Scripts/MIDI Editor/cfillion_Clamp velocity of selected MIDI notes.lua"

Then, you can relaunch Reaper and you should be asked again what you want to do with multiple instances.

I'm not completely sure, but uninstalling > quitting reaper > relaunching reaper > installing should be a working method too.

Also, IIRC, removing the action from the action list, and re-adding it manually resets the flag.

Uninstalling then installing only the script is enough since that removes & reinserts it in the Action List. Relaunching is not necessary.