Date & Time: Most recently, Tuesday November 8, approximately 2000 EST
Frequency: Always, as far as I can tell
World name: Lamia
Character name: Aria Placida
Monster name: Agdistis
Class/Level: PLD 90
Party or solo: Party, of course
In-game time: numerous
Area and coordinates: P7S
Steps:
1. Just before (that is, within about 2-3 seconds) of the first Immortal's Obol cast finishing, activate Holy Sheltron while being first on the enmity list.
2. Get knocked into the air by Immortal's Obol.
3. You will be attacked while being knocked into the air. This attack will be blocked as expected.
4. You will then be attacked just as you are landing. This attack will not be blocked, even though the Holy Sheltron buff that blocks incoming attacks is still active. If you time it really well, Knight's Resolve might even still be providing 15% damage reduction.

This has occurred every time that I've tested it, which is only a handful of times. I'm not certain if the auto attack timings are different for the tank that's second on the enmity list, but it should be easily reproduceable as first on the list.