It is possible to cast guidance (and presumably mark too) with zero mana. The validation only checks whether you are in an encounter when enabling the button.

Is it more surprising that nobody has seen this bug before or that I actually managed to run out of mana?

Probably that you ran out of mana, there have been about a dozen bugs related to bad checks for low mana, but only 2 or 3 times has anyone noticed.

i m abusing 0 mana spells over all limits, mana menagment in strive is rl annoying unless you skip few weeks on nothing but sex