Very Interesting Mana Regen/Cast Speed Bug

I have a stormweaver archmage spark character. Today i bought a new ring and wanted to upgrade my build. However when i equipped the ring, my mana sustain as hold my spark button was cut in half. Here are the rings and other relevant values:



With my old ring, i have 264 mana cost, 4.69 cast speed, 767 mana regen and 4760 mana.
If i hold down my spark button, it takes like 10 seconds to fully deplete my mana (which checks out because i have 1238 mana spend/sec, and i have 767 regen so my net loss is 471 per sec, which would take 10 sec to fully deplete my mana)

With my new ring, i have 272 mana cost (archmage gives 5% of your mana as mana cost so as mana increases so does the mana cost), 4.74 cast speed (Because i get arcane surge effect per mana from stormweaver), 822 mana regen, and 5001 mana.
If i hold down my spark button it takes like 5 second to fully deplete my mana. (Which doesnt make any sense because my previous math would argue that i have 467 net mana loss per sec, and it should take about the same time)

Upon further investigation, i found out that if i take 3% increased cast speed on my tree, the bug actaully fixes itself, and i go back to my normal mana sustain.

The video showcase of the bug:

https://youtu.be/osDattE7QKM

i got same problem please fix!