The first screen of Marlin 2.0 is showing some horizontal lines, appearing to be broken
Something is causing your screen to get signals to display that before your board properly takes it over after 3 seconds.  It is an oddly repetitive pattern.

If it is software, you should not see it on a stable stock marlin release with minimal customizations to enable the LCD.  Usually nothing else shares pins with the LCD, but please do double check pin assignments.

Beyond software lies a realm of hardware issues:

Issue could be due to bad wiring or wires running in areas where they are picking up noise as the system boots up; LCD ribbons are not particularly noise resistant.  I say wiring but it could be anywhere along the circuit, including circuit boards.

There is also some (smaller) chance that your power supply is providing the noise at the initial boot up in which case you may be able to resolve it by adding capacitors to stabilize the DC voltage.  Not sure what check you already did of the power supply, but a DC (RMS) reading may not show anything strange on a regular multimeter even if there is a strange AC signal.