Well, one might assume so. The problem is that it presents an overwhelming advantage and the opportunity cost (even in FoV) of "upgrading" to ANYTHING ELSE for the ring+belt slots would be less than 100 mana and approximately 130 health. Thus, if you use mana with any regularity, it would seem that you lose far more by "upgrading" than you would gain, meaning that a rational player would retain the level 22 ring and level 23 belt for the remainder of the game. This renders all subsequent rings/belts moot and decreases the value to the player of a rich inventory system.

I assumed that it had not been caught by patches because it was a set bonus and assumed that it was indeed an error because having such a substantial benefit from only a pair of items (whereas if reaching that kind of regen from the whole set (or at least many more items from it than simply two) would force you to sacrifice many more benefits from other items to retain the mana regen such that at some point it would be more beneficial to upgrade even at the cost of such a massive mana regen bonus. Even in that situation, however, it still seems disproportionately powerful at that level, since you would be retaining it for that benefit for far longer than you would retain anything else.

As it stands, it is overwhelmingly powerful compared to other options at that level and remains the only rational option for anyone using much mana for the remainder of the game (given its low opportunity cost vs. the substantial investment in skill points and enchantments to even partially compensate for the lost regen.

As it stands, I've been pumping Vitality more than Spirit because, after stacking health regen on jewelry in addition to health and armor enchantments to all armor, I'm effectively immortal and that mana regen means that I can call my creature, summon demon, and sling infinite spells with no trouble.

I suppose it might not be an error, but I do not think it would be a prudent game design decision and the quality of the rest of the game led me to assume it was a mistake rather than what would otherwise seem to be an indisputably bad design decision.

And thank you for the welcome.