I updated the firmware S10 5G yesterday and I stopped working fast charging. (It's not a matter of charging, I tried something else) sometimes it does and not work sometime
Can you clarify have you used another Samsung Charger that has fast charging capabilities ?
Can you try a wireless fast charger.
Sometimes a soft reboot by turning the phone Off and On helps.
As a last resort back up and remove any Sd card and factory reset.
A Samsung Service Centre or Samsung Experience Store can run a diagnostic test on the phone.
Current Phone ~Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+ Samsung One Ui 3.1/ Android 11.