Encountering this bug now. I just did some testing on my assassin, because she has no mana start gear, just 1% from citadel. I am using Nightmare vision (75), Unnerving Strike (120), Channel Darkness (50) and blackjack (75).
For me, what happens is this…
Prior to making a match:
Nightmare vision - 1/75, Unnerving Strike - 1/120, Channel Dark - 0/50, and blackjack 1/75
If I make a 3 match of dark gems:
Nightmare vision - 50/75, Unnerving Strike - 50/120, Channel Dark - 49/50, and blackjack - 50/75.
If I open with a 4 match of dark gems:
Nightmare vision - 67/75, Unnerving Strike - 67/120, Channel Dark - full/castable, and blackjack 67/75.
If I open with a 5 match:
Nightmare vision - 1/75, Unnerving Strike - 83/120, Channel Dark - full/castable, and blackjack 1/75.
If I open with a 6 match:
Nightmare vision - 1/75, Unnerving Strike - 99/120, Channel Dark - full/castable, and blackjack 1/75.
Finally, if I get a 7 or more match:
Nightmare vision - 1/75, Unnerving Strike - 1/120, Channel Dark - full/castable, and blackjack 1/75.
Channel dark does not appear to get buffed by starting mana via the citadel, some other testing shows that none of the non-class spells do. (That seems like it’s own separate bug.) As such, it always fills normally and I can cast it. If I cast it, all of the “bugged spells” become castable immediately.
One last thing, while the non class spells are not getting starting mana from the citadel, they ARE getting starting mana from gear.