I don't work in ecology, but my first thought is that I would not expect any relationship whatsoever between diversity and biomass. Biomass simply means the combined mass of all life on the planet. If that mass consists of one extremely fat goat the size of the moon, or several trillion different organisms doesn't make a difference, mass is mass.
Now, it is undoubtedly true that loss of diversity is a huge issue. However, the way this works is usually that we replace the local ecosystem with its several, varied species with a monoculture. For example, grasses like wheat and barley are taking over the world. Destroying a forest to plant wheat might easily result in no net change in biomass or even a net positive, but it will certainly be a significant loss in the diversity of the local ecosystem.
Finally, humans are a teeny tiny part of the world's biomass, most of which consists of unicellular life, plants and insects:
Note that the axes in the image above are logarithmic. You can get a clearer image, perhaps, from this:
(both images taken from Bar-On YM, Phillips R, Milo R., The biomass distribution on Earth, PNAS, 2018 Jun 19;115(25):6506-6511. https://www.pnas.org/content/115/25/6506)
See how all animals, let alone humans, are just a tiny part of the image on the right? That gives you a pretty clear indication of just how irrelevant we are in terms of global biomass. So yes, diversity is indeed decreasing, but there is no reason to think that the accumulated mass of living organisms will decrease, only the diversity of said mass will decrease.