iPhone 13 liquid damage despite IP68 certification

Your phone is not waterproof, it is water resistant according to the IP68 certification. This is a controlled laboratory test, not dropping a device into water. The splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. All stated very clearly by Apple here:

About splash, water, and dust resistance of iPhone 7 and later - Apple Support

The IP68 rating is under lab controlled conditions. Not real world conditions. And over time, phones lose their water resistance as the phone ages. If water got in your phone, it can't be fixed, so you'll need to purchase a new iPhone.

I dropped my iPhone 13 in a pool

The IP68 Certification says nothing about dropping a device into water. For IP68 tests, a device is placed in a mesh type basket or tray and then slowly lowered into calm water. Then, the device is slowly raised back to the surface.

You can ruin an iPhone in a few seconds by dropping it in a toilet, as my wife discovered recently.

It's possible because it is water resistant, not waterproof.

The specifications are based on laboratory testing under controlled conditions.

In addition, your actual phone is not certified, the manufacturing process is designed to turn out phones to that standard. Some don't make it, hence the small print "Water resistance is not guaranteed and can reduce over time"