Unclear on any display issue. Fastboot is just an appsboot stub.
If you didn’t clear partitions, and cross flash bins you may have inadvertently made your own breaking changes. If you look at the flash all script it wipes out partitions. So a stale partition “could” cause a display issue. If you’re confident you did the right thing with your partitions, then the issue is elsewhere. Again no idea about display issue you’re referencing.