This affects the WikimediaDebug "Inline profile" feature, which is backed by MediaWiki's ProfilerOutputText and ProfilerXhprof classes.
Example from https://en.wikipedia.org/w/load.php
1152600.00% 11.526 10 - Wikimedia\Services\ServiceContainer::getService 1151300.00% 11.513 4 - Wikimedia\Services\ServiceContainer::createService 1146100.00% 11.461 4 - Wikimedia\Services\ServiceContainer::{closure} 1046500.00% 10.465 1 - MediaWiki\MediaWikiServices::getResourceLoader 688600.00% 6.886 131 - ResourceLoader::register 594700.00% 5.947 1 - ResourceLoader::respond
Note how the percentage of main() is not 100% as it should be. They all seem to just be inflated copies of the millisecond measure.
Upstream: https://github.com/tideways/php-xhprof-extension/issues/103