Oh yeah currently propto = TRUE is only set to true for things using the sampling syntax. There’s a discussion over here of extending the rules so users can take advantage of the proportionality thing manually: Request for final feedback: User controlled unnormalized (propto) distribution syntax
I think this still may be an outstanding issue with propto handling: https://github.com/stan-dev/stanc3/issues/541#issuecomment-632003010