Make sure there's absolutely no way skeletons can shoot into the pen. They will kill every single boar. Have workbenches on and around the pen to prevent night time mobs from spawning in. As for wolves honestly unless you dedicate to making an all stone structure with roof there's no protecting them from frostdrakes in the mountains, not worth taming IMO.

I'm not sure how often it happens, but I have seen an issue where structures load in AFTER mobs do, in online servers, allowing mobs to move in or tamed animals to move out of it, before they become solid. That was an older bug though that happened to me before but hasn't since.