If you're using one of these computers and also use Pixelmator Pro, it would be best to hold off on updating to 10.14.3.
We'll update this thread once we have any news about potential improvements or fixes, either from our side or in macOS.
If you have any other questions, feel free to post them here or you can also always reach us at firstname.lastname@example.org.
Update (02/05): We're hearing that a potential fix is on the way, possibly in time for the next beta of 10.14.4.Affected models:
2016, 2017, and 2018 MacBook Pro (with integrated GPUs only)
2018 Mac mini
2018 MacBook Air
Update (02/06): We've now found a workaround of our own and released it in our public beta of Pixelmator Pro 1.3.1. You can join that beta program here. This version should go live on the Mac App Store at some point next week.