I am using the same adapter, with an LG 24UD58 and with Dell P2415Qs. Have you tried a different HDMI cable?
For a second I had my hopes up - I did order fresh cables from AmazonBasics, but what if they were not up to spec? So I tried a different cable, a CSL HDMI 2.1 (rated 4K@144), but nope, same thing. I also just moved my PC next to the TV to test with, output driven by a Nvidia GTX 980 - AmazonBasics cable: 4K@60 works. CSL cable: 4K@60 works - not the cables, not the TV.
EDID export from SwitchResX attached.
My next test will be ordering one of these active converters: Club3D CAC-1504 active USB-C to HDMI 2.0 adapter.