It happened when I was also in melee combat with a melee guard from a tower.
Afaik having a guard attack you in melee combat should not remove your large shield blockrate bonus vs projectiles.
With large shield my blockrate is around 50% vs projectiles, without its around 20%. In the picture below you can clearly see it being way lower while the guard attacks me. The guard was also the only melee attacking me, thus there should be no malus from having to many attackers. Especially because the first arrow was being blocked with an already low %, before the other archers attacked me which would result in a total amount of 2 attackers. To me it is clear the 30% bonus was not applied because I was in melee combat with a guard. I faced the attackers (both guard and archers)