I . M facing heating issues with one plus 9
Also read this about 188.8.131.52 and upcoming OTA addressing heat and battery (first post)
184.108.40.206 for 9 Pro and 9 released today, install it when it presents itself, go through 2 full charge cycles, restart the phone after the 2nd charge cycle and see how this works out for your phone.
Wait for updates
Try changing a case for oneplus 9, there are some cases that cause overheating on the devices use.
Update software version
India Needs You. Let's come together in support
by Pete, 2021-04-30