I have Percival and couple other SK cards. When I play them one after one (no other cards on board), he does not get +2 boost.
Did you mean you played it with other Scoia'tael cards? It never works with units from other factions, like Skellige. Percival Schuttenbach's Harmony ability only triggers when a Scoia'tael unit with a unique tag among all units on your side of the board is played in the same row. Harmony was always terrible in Arena, but with the recent nerf to Harmony, I would avoid that keyword like the plague when drafting your deck.
Well, if you didn't have another Human on your side of the board, then yes, that does sound like a glitch.