Intended Interaction of Mana remnants + Mana Tempest?

Currently, if I use Mana tempest when I do not have overflow mana (no mana remnant pick ups), Casting Freezing Shards does not increase the mana cost of Mana tempest, and my mana regen can sustain it

But if I have overflow mana ex: 1203/1000 mana, my Mana Tempest mana cost ramps up very quickly despite not casting anything while in it.

Is this intended?

https://streamable.com/8vjkxh -> no overflow mana
https://streamable.com/o5boig -> with overflow mana

(links expire in 2 days just in case they go down)