Armor bug or works as designed? (not the same as armor is 0)

Hi, i’ve noticed a few days ago - if i join the group, my armor is reduced (e.g. on solo play i have 1,2k armor → after joining the group it reduces to 800 → after leaving the group it’s at 1,2k again)
Is it a bug or does it works as designed? i was not able to find any info about armor debuff in groups…
it’s not the same bug as already discussed here with armor=0