fix: use userExists instead of domain equality

--- ./lib/private/User/Session.php	2026-03-23 17:46:39.585001447 +0600

+++ ./lib/private/User/Session-new.php	2026-03-23 17:59:52.577453313 +0600

@@ -384,6 +384,14 @@

@@ -384,6 +384,12 @@

 		$remoteAddress = $request->getRemoteAddress();

 		$currentDelay = $throttler->sleepDelayOrThrowOnMax($remoteAddress, 'login');

+		$mainDomain = $this->config->getSystemValue('main_domain', '');

+		$legacyDomain = $this->config->getSystemValue('legacy_domain', '');

+		$mainDomainSuffix = !empty($mainDomain) ? '@' . $mainDomain : '';

+		// Don't replace legacy domain in login name

+		if ($mainDomain !== $legacyDomain) {

+		// If user with given UID exists, we don't want to replace the domain

+		if (!$this->manager->userExists($user)) {

+			$user = str_replace($mainDomainSuffix, '', $user);

+		}

+

 		if ($this->manager instanceof PublicEmitter) {

 			$this->manager->emit('\OC\User', 'preLogin', [$user, $password]);

 		}