Does it only affect the lock screen or the home screen too?
Does the bug occur in the owner profile or another user profile?
The phenomenon could be a known upstream bug:
https://github.com/GrapheneOS/os-issue-tracker/issues/1461
If you search for "stretched wallpaper" in the forum, you will find a few posts about it, such as this one:
https://discuss.grapheneos.org/d/715-wallpaper-stretching-bug-on-p6pro-after-last-2-updates
I also had the problem for some time (in my case it only affected a user profile, not the owner profile and occurred on the home and lockscreen) and then used a monochrome wallpaper for a while. A few weeks ago I tried again with a different wallpaper and now it is no longer distorted - unfortunately I can't tell you why.