Do you also have Dark Mode chosen in the Messenger app? Messenger has an additional Dark Mode setting that will change its appearance and you want to have it set to System to respect the setting you have on your device.
https://www.facebook.com/help/messenger-app/2137290196561574?cms_platform=ipad-app&helpref=platform_switcher
Previously they had the same problem with the Dark Mode setting in their app and it was resolved with a Messenger update. That may be the case here too. Meta did update Messenger 2 days ago, along with the 11 they have had in just the last month. Some update include bug fixes and others introduce new bugs that need fixed in the next update. Meta has had the beta version of iOS 26 for over 3 months to test their features for any compatibility problems. Any resolution would come from Meta.