If a wiki is configured to use Monolog (or any other PSR-3 logging system other than the default MediaWiki\Logger\LegacySpi) calls to wfDebug, wfDebugLog and direct use of named PSR-3 loggers will not be passed to MWDebug::debugMsg() for output as HTML, comments or Debug Toolbar data. Generically in the PSR-3 standard there is no way to handle MWDebug::debugMsg()'s expectation of receiving a copy of all log events.
MediaWiki\Logger\LegacyLogger (the current default PSR-3 implementation) forwards to MWDebug::debugMsg() for all calls LegacyLogger::log(), even if the log event will not otherwise be forwarded to an output channel. MediaWiki could provide a class implementing Monolog\Handler\HandlerInterface that could be bound to any/all logging channels and forward log events to MWDebug::debugMsg(). Instances of this special handler class could be automatically added to each logger acquired from MediaWiki\Logger\MonologSpi::getLogger() to replicate the legacy behavior.