I just installed PixPro 3.4 and found previous first image open slowness seems to be resolved. While I was confirming that I decided to test opening multple .pxd files and see how it was handled. I chose 15 different sized .pxds in Finder, to open them with Pixelmator Pro. As the first image shows I had 15 files selected but the second image confirms that it only opened four, and also interestingly it did not open any of the four in contiguous selection order - it appears random. The four open files were a total of 54.9 MB on my Apple M1 Max, 32 GB.
I wonder why that is? What are the application work space limitations?