[thejeff]

There's a known bug with Astral Shield having a higher priority than it should, especially with high Astral mages.
Supposedly there's a fix in the upcoming patch.

You can, to an extent, work around it by looking at the debug log for a battle and figuring out which of your other spells will not be overridden. The bonus for buffs that puts Astral Shield over the top is larger earlier in the fight, so you can sometimes get the desired spells off, if you shift the order around.

Hopefully, after this weekend, it won't be an issue.