I have same problem. My Synology in extreme situation switch down for overheating on 61°C, ambient temperature 31°C.
Today I have measured ambient temperature, it was 24,6°C, on the top of Synology was 36°C and disks were from 40°C to 56°C in direction to mainboard. When fan start to run, than disks temperature is from 39°C to 41°C.
I tried to change fan control, power sawing, silent, cool, but nothing changed.
Seems to me as a software bug.
Only solution I see, not to use hibernation till Synology fix the bug.
DS411slim, 4 x WD10TPVT-00HT5T0