To reproduce:

1. Get a wild wolf, for example: /summon wolf
2. Hit it in Survival mode.

Result: Wolf becomes angry at you and tries to attack you.

3. Leave and rejoin the world (only tested in Singleplayer, might require a server restart).

Result: Wolf is peaceful again.

Running /data get entity @e[type=wolf,sort=nearest,limit=1] reveals that there is no NBT tag for who the wolf is angry at, only Angry:1b or Angry:0b.