Arrow damage when shot from a bow is roughly calculated in the following way:

baseDamage * velocity + (if bow fully charged) randomCritDamage

baseDamage  starts at 2 for an unenchanted bow. In versions before 1.21, Power increases this value by 1 at level 1, and increases the value further by 0.5 for every subsequent enchantment level, making the maximum base damage 5 at level 5.

In 1.21 and above, the value increase at level 1 is only 0.5, and still increases further by 0.5 for every enchantment level - making the maximum base damage 4.5 at level 5 - lower than what versions below 1.21 allowed.

To fully verify this, I made a small Fabric mod for 1.20.6 and 1.21.1 that prints out the damage values used in the calculation in order to compare them, using a bow enchanted with Power 5. In the attached screenshots, it can be observed that the base damage values between versions are, in fact, different.

Likely the reason for this issue is that in the .json file that contains data for the Power enchantment in 1.21 and above, the base damage increase is set to 0.5 and not 1.0.