Has anyone seen an issue with Late 2015 27" iMacs where the screen will flash black once at the login screen? The machine is running 10.14.4 but the issue also occurred on 10.13.6. The machine will boot to the login screen then when the user starts typing their username it flashes black and clears it out. It only happens once. I have removed all the login configuration profiles thinking that might be part of the issue, however, it is still happening. Anyone have any thoughts for troubleshooting? AST came back okay.