It is normally splash proof, water-resistance is even part of the EASA/FAA requirements for aircraft of that weight class. It sounds like a technician failed to install the foam/plastic sealing layer when they last serviced the ACPs. Or the pilot might have spilled a lot more liquid on it than reported.
I interned at Thales's returns department when I was in school getting my Electrical Engineering degree (The group I interned with evaluated returned avionics to identify common failures). We received a lot of returned avionics systems that were missing their seals even though we include 2-3 in each of our service kits.