JSFX (and perhaps other plugins too, I don't know) can define a "center" value for their parameters. This center value need not be the arithmetic average of the minimum and maximum of the range: For example, a volume parameter may range from -120dB to +10dB with a center value of 0.
In the envelope lane, REAPER will display this center value in the middle of the lane. (There may be Preferences to toggle this on and off.) Therefore, if the center value is not in the middle of the range, you will get a twisted line, as in your screenshot.
Envelopes looking strange?
Linear Mode
