MacBook Pro 13" 2019 won't display @ 75hz with 3800 x 1600 resolution (Acer XR382CQK)
I've seen this issue come up all over the internet so I decided to do a QA with a solution I stumbled upon by chance

Disabling USB 3.0 allows me to use the monitor at 75hz.

This only affects peripherals plugged into the monitor dock.

On the XR382CQK the option is under Menu > System > USB 3.0

I haven't had a chance to explore why this is (my theory is something with bandwidth limits) but turning USB 3.0 off allows up to the full 75hz refresh rate to be used, meanwhile turning it on instantly downgrades the refresh rate to 30hz again.