Int to mana bug?

[NPC]

I've been noticing int giving different amounts of mana above 220 int? Is there soft caps? I've not seen any info on that.
So I have a 49th level wizard, his current items he has 224 int, plus some mana add items for a total of 2351 mana.
I take off the 5 int Mystic Cloak and my mana goes down to 2333.

If I take off my loam encrusted sleeves adding 5 int, 5 mana, put on my embriodered black sleeves that gives me 25 mana add, so I'm at 219 int, putting me at 2353 mana. Then i take off my 5 int Mystic Cloak and my mana does down to 2323.
So under 219 int the 5 int gives me 30 mana, but when I'm over 220 int, it only give me 18 mana! Thats a pretty big difference, I bought the Loam sleeves thinking they would give me more mana than my embroidered black sleeves, but my int so high it actually gives me less? How the hell am I suppposed to know if an item will increase my over mana like this? Are there more caps the higher your int goes? Decreasing the int per mana further?