This likely also impacts the normal Ferrox.
Reproduction steps:
1. Start a mission as Citrine
2. Deploy the Prismatic Gem ability (3)
3. Throw a Tenet Ferrox field using alt-fire at some enemies
Expected: When enemies are hit by the pulsing Tenet Ferrox field, the Prismatic Gem fires Prismatic Beams at them.
Actual: When enemies are hit by the pulsing Tenet Ferrox field, the Prismatic Gem does nothing.
Explanation: The Prismatic Gem should be firing when enemies are damaged by weapons, as per the description, and it does seem to work correctly even with another area deployable (Zenistar's disk), but not the Tenet Ferrox fields. This inconsistency is almost certainly a bug.