The default monk gloves provide an attack bonus rather than an enhancement bonus so it's plausible that the additional damage being dealt by the mace may be just enough to overcome the bludgeoning resistance, if it is indeed part of the creature's skin. If the creature is not crit immune (like a construct or undead type), just a small amount of damage could be multiplied, thereby appearing to be combat-effective. The name Glacial Juggernauts means nothing to me and I am not familiar with any of the default creature blueprints that specifically include bludgeoning resistance (perhaps MM knows of some?), so if it is indeed factoring in, it would be a custom skin. Or the designer may just have stuck one of default bludgeon-resist belts into the monster inventory. Lots of possibilities any way you look at it.
A lot of monk gloves provide elemental damage, so it seems strange that at least that portion isn't making a dent (if the gloves have such a damage enhancement, that is).
Another option could be some percentage of bludgeoning immunity that the mace's damage enhancement may be just high enough to cause some damage. I remember some mummy encounters where that immunity was an issue. Jellies also have general physical immunities as well.
But I am just guessing. Knowing what creature type the custom monster is based on might help figuring the source of ineffectiveness of the gloves better.
I'd be interested to see what a more mundane (less than +5) kama would do in the same battle, just to narrow down what the monster characteristics might be.
