Just for info before I start here,i already sent an email to support@, before noticing this lovely support forum here. Sorry about that, but as someone else might also stumble across the same behavior, I believe it might be worthwhile also posting here.
I have just started integrating (more initial testing, really) the Hyperpixel 4.0 on my 3B+ and Zero W. Both with the same behavior of the screen.
Both Pis have a fresh Raspian install and the Hyperpixel SW from github.
The display fires up on power on, but shows quite a few white lines, that are deriving from bright pixels and are spreading all across horizontally.
They are well seen on the CLI, but also shown on the desktop spreading out e.g. from the cursor.
I am wondering whether this relates to some driver / settings issues, or if there is some hardware faulty (e.g. a cable not 100% secured(. I did look atthe two flex-PCBs, they seem to be correctly in place.