I used the same work-around as akosnitzky.
At first, it wasn't so bad: calendar invites were sent from my
address, then suddenly* I noticed that they were being sent from another address, for which each meeting invite had a big red X. I could resend it by changing the from-address at that point, but it meant always having to do that after a failed send from an address which didn't work. Then, seemingly as suddenly, it was sending from another address again; this time it was working except it wasn't the address I wished to send invites from.
*it wasn't so sudden, but it took the third one to figure out the pattern. Each time I added a new address to BIS, it became the default. I ended up deleting the CICAL service books from every address except the one I wanted calendar invites from, and it worked. I am not sure what will happen if I resend calendar books from the BIS website (will it change the default?) but I know what to do about it now.