If an object is on the edge (but fully within) a stationary shield it will take damage when an ACU or seraphim SACU overcharges a target underneath the shield. Non-stationary shields do not seem to be affected (Harbingers and T2 mobile shields tested so far).
Video of ACU damaging T2PD underneath UEFT2 shield: https://streamable.com/mqa4so
Video of overcharge damaging T2PD underneath UEFT2 and T3 stationary shield: https://streamable.com/bpfwgu
Ground POV of units being damaged underneath shield: https://i.imgur.com/gRkDILW.png
Replay IDs used for testing: #13637823, #13631504