This happens off and on.
The mirror shield will reflect one fireball and then absorb the next, leave the screen, come back, and it works normal, go to another screen, it does it again.
mirror shield absorbs fireball
This is still happening in 443
It seems to happen when link is sideways and the fireball is at a steep angle coming down on him.
They were getting reflected off of the shield, and then immediately absorbed.
Fixed in the next build.
Reflecting sharp-angled projectiles looks silly in a lot of cases - the fireball passes clear through Link's ass without him taking damage - but I think that's the way the shield has behaved since 2.10.
Confirmed fixed in 564
Dead Bug!