I'm using exactly the same usb-C to HDMI adaptor: when I plug it into the MacBook I get a maximum resolution of 1920x1080 and when I plug it into the MacBook Pro I get the correct resolution of 2560x1440. Both laptops are running OSX 10.14.4
The MacBook also incorrectly identifies it as a 30" monitor. I tried resetting the SMC and NPRAM, no difference. I tried with SwitchResX, and when I select 2560x1440 it tries to push all of those pixels onto 1920x1080 (I can read the actual current resolution off the monitor's inbuilt menu) and the image is all blurry.
I tried everything I could think of and I'm out of ideas.