Probably caused by HDR tone mapping. Native PMS on Synology does not have necessary drivers to do HDR tone mapping using GPU and so video transcoding is done by GPU and tone mapping by CPU. To utilize GPU for both video transcoding and HDR tone mapping, You need to use docker linuxserver.io PMS version which have all drivers.
So first try to turn off hdr tonemapping in PMS settings to make sure that the problem still occurs.
