Could be multiple reasons. Your cable could be damaged. Try first flipping it both ways. If it doesn't fast charge on one of the sides, buy/borrow another cable. Slow charging could also be due to heat. If you live in an environment that has around 35 degrees, then charging will be slow to protect the battery. The best thing is to turn the phone off completely while charging in a super hot environment like this.
Yes, After so many users posting the same problem (charging issue), still not received an correct answer or solution. Just think so many people facing the same problem and not receiving an correct solution. How can we trust this brand anymore and recommend it to others. I lost the hope on this ASUS
I have replied in multiple threads like this one and other users have said what you're saying but in all cases it has proven to be either damaged cable, charger or phone. Not software. This is common assumption to make because you see other users with the same issue and it's more convenient to believe that something can be fixed with an update. This is a very common issue on all phones because there are many multiple reasons why it can happen. If you're so sure it's software, then you're welcome to downgrade to Android 9 and see if it fast charges again (you will lose all data so backup first). The downgrade is available on our home page and some have already done this test only to find out that software wasn't the issue. I recommend you to visit ASUS Service Center