Game v1.1.11. Scavenger difficulty. Observed and repeated during random encounter with apprentice necromancer and 3x zombies.
Character affected is a level 4 Shadowcaster Rogue wielding two shortswords and has the Twin Blade feat. Shield spell was selected at level 3.
The rogue is level 4, and is defending against a melee attack that exceeded AC by 0-2.
No reaction prompt to use twin blade occurs as long as the character has available lvl 1 spell slots.
What is interesting about this is that when the attack occurs there is the message in the combat log saying that shield reaction could not be cast due to not meeting somatic requirements. This is obviously expected (the shadowcaster does know the shield spell but does not have a free hand to cast), but I should still get the twin blade reaction as far as I am aware.
However, if no spell slots are available (all used), then the Twin Blade reaction does trigger.
So it seems to be something with the shield reaction being checked (even if unable to be used) that is blocking the Twin Blade reaction from triggering.
Also repeated test with somatic requirements turned off in difficulty. In this case, Shield would trigger (as expected) and could be used, but if it was not used there was no follow up prompt to use Twin Blade instead. However, again once all uses of shield were expended, Twin Blade would trigger as expected.
Finally, also tried a level 4 green mage with dual daggers who also knew Shield and had Twin Blade using one of the test dungeons. Same behaviour was observed.