notices and warnings displayed with WP_DEBUG false

My WordPress sites suddenly started displaying notices and warnings publicly. even when WP_DEBUG is set to false. Even when I set it to true I normally do WP_DEBUG_DISPLAY false. Either way, sites are displaying all sorts of deprecated function notices and other info I dont want shown publicly.

Tried reverting to WP 5.7.2. Contacted Siteground to ask if it could have something to do with php upgrade, but rolling that bac didn’t help r ither.

I run some custom plugins, so could be my fault somehow. But I dont understand how my code could overwrite a WP constant. Would appreciate any suggestions for what to check next