Hmm
I poked around a bit, and this is definitely a bug, however it seems to be some very elusive issue…
The cause of the damage I found out, that part is clear 
What’s happening is that under certain conditions the Shield Master 4x block cost is applying to the health damage type as well, which it should never ever do
As you can see on that image, it’s hitting the players with quite the damage because of the 4x multiplier and they also get this for stamina ofc.
The elusive part is… why is this only happening sometimes… in the short time I looked, I couldn’t find any reason for it… and a lot of times the block is perfectly normal, the only way I can trigger it is by repeatedly restarting PIE (play in editor / the game basically) and then when I manage to get it, it will do it every single time without exception.
Hence why I think it’s either some weird timing issue… or something weird happening in the parts that I can’t see…
However what’s happening is that in these cases, we get the full damage multiplied by that 4 (not the shield block reduced damage - which should ideally be 0% for health and like 8-9% for Stamina) so… this part of the code would most likely prevent it, if an OR “Not has the CanBlockAllPerk” would be added to that condition, since then it would always run the shield mitigation code after it when blocking with shield master. I still don’t get what’s mitigating the damage when it just decides to work and doesn’t run into that part of the code… cuz based on this… it should always do this… it’s… magic
This is just shooting in the dark 
but maybe it’s helpful…