Perhaps there was a recommendation to suddenly keep these deer on bark chip, who knows, but the factor that precipitated it seem to be the introduction of the pigs.
I rather doubt it. Other groups in the UK still live on grass. I think they did the mixed exhibit and the Warty Pigs then ruined the grass paddock quickly, so they redid it as a bark-chip paddock. I felt sorry to see the Deer now with no grazing whatsoever- though they do provide browse in the form of tree branches I think. Its also interesting how this group of Phillipine Deer seem to have remained constant at 1.3. for a long time, with no evidence of breeding.
I would prefer to seem the two species kept as Newquay does, adjacent but not together so that the Deer's paddock remains intact.