https://forum.xda-developers.com/galaxy-s9-plus/themes/utility-amoled-pwmfree-application-t3898070/amp/ Install this application and it will stabilise the display... (PWMfree)
I am also having the same issue after the March update...
I almost tried everything to fix it... but still the same when the brightness changes....
And I don't think it can be fixed by a software update.. probably it's a hardware issue..
