When I prep the spell, it says I'll have enough mana. But then when I try to cast it, I'm told I don't have enough. I haven't done anything in the meantime to lower my attunement.

>prep soul
Since you're not feeding enough power into the spell pattern to make it coherent, you quickly work your way to the minimum required.
That will disrupt most of your current attunement!
However, if you utilize a ritual focus: That won't affect your current attunement very much.
You trace a careful sigil in the air, shaping the pattern of the Soul Ablaze spell.

You feel fully prepared to cast your spell.
>cast
You relax your eyes in order to focus on the trace amounts of Fire that warm you and the world around you. Your skin glows faintly to your relaxed vision and your breath puffs forth like luminescent steam.
You strain, but are too mentally fatigued to finish the pattern, and it slips away.


(Before someone posts this: yes, I know how ritual focuses work. This isn't a post about those.)

> Can you duplicate this issue with a non-ritual spell?

No, but that's because there's no non-ritual spell that can use up all my mana.

> Check your concentration.

Full both before and after. "too mentally fatigued" is the message for not having enough attunement.