Switched from Pi 3B+ to Pi 4 - random lights at boot

Today I replaced my Raspberry Pi 3B+ with a Raspberry Pi 4B. My matrix display is connected to the first chain (the Smiley :smile: configuration on the wiring howto) - same as it was on the 3B+.

When I boot my Raspberry Pi 4, a row of LEDs lights up on boot and it stays on until I actually start displaying something. The same also happens on every reboot. This did not happen with the older Pi.

This is not a showstopper in my case, but if anyone knows an easy solution (or can give me hints on how to further debug this), Iā€™d be all ears!