I always had this problem but with an NVIDIA GPU, GRUB and my MSI monitor (and I guess still now with systemd-boot). Switching to HDMI solves my issue as well. I made this (very confusing) thread a while ago trying to solve the issue. But I could never fix this problem.
But for you it seems like it worked with GRUB but stopped working with systemd-boot?
For now my solution is to 1) turn on my monitor before I turn on my PC and 2) I increased the /efi/loader/loader.conf timeout time by a bit and spam Arrow Down & Arrow Up when I boot so that I am still in the select boot screen when my monitor finally displays a signal over DP.