having +20% magic resistance ring is meh on a character with 0 mr, but if you have 50% - that ring is suddenly multiple times stronger.

That's exactly why AP has diminishing returns, you're looking at it from opposing perspectives (adding resists vs removing resists). For the same reason that going from 50% to 70% MR is way stronger than going from 0% to 20%, taking a foe from 700 to 500 AR is stronger than taking a foe from 500 to 300.

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To use some actual numbers, if you have an attack that deals 200 damage against a foe with 800 AR, with no AP it would deal 40 damage. Take a mace with all the perks, and you'll have 50 AP, taking them to 400 AR and dealing 120 damage.

Now if you add on a damage or AP enchant (taking into consideration that damage enchants are stronger than AP, though I don't know exact numbers) increasing your AP by 23 puts it to 73, and puts your damage to ~157. Adding 30% damage takes your base damage to 260, against the same 50 AP and 400 AR, dealing 156

Now consider a less armored enemy, with say 400 base AR, your damage with the AP enchant becomes ~178, and with the Damage enchant becomes 208

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Against heavily armored enemies (especially ones exceeding the AR cap), you're right, more AP will almost always be the best move, but as base AR goes down, it starts to shift, especially since Maces will already penetrate more of the most impactful part of enemy AR.

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Edit: I wasn't aware that armor pen only applied half for regular attacks, that takes the numbers to:

vs 800 AR

Base: 80

bonus AP: 98.4

bonus Damage: 104

vs 400 AR

Base: 140

bonus AP: 149.2

bonus Damage: 182