[INFO] Another Tonemapping Discussion. (FFmpeg)

[Selur]

but using zscale=p=bt709 before tonemap=tonemap=mobius caused such issue.

Yes, but doing that makes no sense at all.
Tonemapping is not meant to be applied on bt709 content, it seems to me it's something like applying deinterlacing filters on progressive content and complaining that the quality got worse.
It's something that may make sense in some specific cases, but if you blindly do stuff you can have issues.

Cu Selur