With SSAO on, there's these weird black artifacts across the screen as shown here, normally while next to a block. Please note that this happens with version 1.3.7 with Fabric 1.21.4 & the latest nightly beta of Distant Horizons from the DH Discord. Look very closely at the excessive darkening:

If it helps, my system specs:

CPU: Intel i5-1135G7
GPU: Intel Iris Xe Graphics
RAM: 8GB RAM
OS: Linux Mint

similar problem, i thought it was player shadow.

edit: fixed by disabling anti aliassing