1. Download and unzip the following archive: https://db.tt/xus3dlRS
2. Copy the two files "debug_anticlockwise" and "debug_clockwise" to "~/Library/Application Support/REAPER/Effects" or the equivalent folder on Windows
3. WARNING: Turn down audio levels before opening "Parameter Modulation Bug.RPP", as this project will produce pink noise as soon as it is loaded.
4. Start audio playback, and observe parameter modulation on the two tracks.
The track "Clockwise - OK" uses the "debug_clockwise" plugin. This plugin has the angle parameter defined as follows:
Parameter automation works as expected for this parameter.
The track "Anti-clockwise - BUG" uses the debug_anticlockwise plugin. This plugin has the angle defined as follows:
As you see, this plugin is making use of the new feature introduced in Reaper 5.0 that JSFX parameters can have their range inverted (increasing to the left). If parameter modulation is disabled, you can play around with this plugin and it seems to work fine. However, I am not able to find any Parameter Modulation settings that produce circular movements in the GUI. Either it will be stuck to the left (180 deg), to the right (-180 deg) or it will jump back and forth between these two extremes.