diff --git a/inc/admin-pages/class-multisite-setup-admin-page.php b/inc/admin-pages/class-multisite-setup-admin-page.php
index 10d994462..6a6d618eb 100644
--- a/inc/admin-pages/class-multisite-setup-admin-page.php
+++ b/inc/admin-pages/class-multisite-setup-admin-page.php
@@ -484,6 +484,17 @@ public function setup_install(): void {
 		$installer                   = wu_request('installer', '');
 		$multisite_network_installer = Multisite_Network_Installer::get_instance();
 		$steps                       = $multisite_network_installer->get_steps();
+
+		/*
+		 * Only handle installers that belong to the multisite network installer.
+		 * Unknown step names fall through to the next wp_ajax handler (priority 10)
+		 * which handles Core_Installer, Default_Content_Installer, etc.
+		 *
+		 * IMPORTANT: this must use `return` (not exit) so the Setup_Wizard_Admin_Page
+		 * handler can process its own steps. But we must NOT let a known multisite
+		 * step silently pass through — if handle() returns a non-WP_Error the step
+		 * genuinely succeeded; only unknown steps should fall through.
+		 */
 		if ( ! isset($steps[ $installer ])) {
 			return;
 		}
@@ -492,6 +503,8 @@ public function setup_install(): void {
 
 		if (is_wp_error($status)) {
 			wp_send_json_error($status);
+
+			exit;
 		}
 
 		wp_send_json_success();
diff --git a/inc/admin-pages/class-setup-wizard-admin-page.php b/inc/admin-pages/class-setup-wizard-admin-page.php
index af873d272..6fa53152e 100644
--- a/inc/admin-pages/class-setup-wizard-admin-page.php
+++ b/inc/admin-pages/class-setup-wizard-admin-page.php
@@ -153,8 +153,11 @@ public function __construct() {
 	/**
 	 * Handles the AJAX request to network-activate Ultimate Multisite.
 	 *
-	 * Attempts to network-activate the plugin, returning a JSON response.
-	 * On success the caller should reload the page so the checks refresh.
+	 * Writes directly to the sitemeta table instead of using activate_plugin()
+	 * because is_multisite() may return false when OPcache serves a stale
+	 * wp-config.php. activate_plugin() silently falls back to single-site
+	 * activation in that case. The direct sitemeta write guarantees reliable
+	 * network activation regardless of bytecode caching state.
 	 *
 	 * @since 2.3.0
 	 * @return void
@@ -169,14 +172,10 @@ public function ajax_network_activate(): void {
 			exit;
 		}
 
-		if ( ! function_exists('activate_plugin')) {
-			require_once ABSPATH . 'wp-admin/includes/plugin.php';
-		}
-
-		$result = activate_plugin(WP_ULTIMO_PLUGIN_BASENAME, '', true);
-
-		if (is_wp_error($result)) {
-			wp_send_json_error($result);
+		try {
+			Multisite_Network_Installer::get_instance()->_install_network_activate();
+		} catch (\Throwable $e) {
+			wp_send_json_error(new \WP_Error('activation-failed', $e->getMessage()));
 
 			exit;
 		}
diff --git a/inc/installers/class-base-installer.php b/inc/installers/class-base-installer.php
index fa578b402..84013793b 100644
--- a/inc/installers/class-base-installer.php
+++ b/inc/installers/class-base-installer.php
@@ -99,7 +99,14 @@ public function handle($status, $installer, $wizard) { // phpcs:ignore Generic.C
 			return new \WP_Error($installer, $e->getMessage());
 		}
 
-		$wpdb->query('COMMIT'); // phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
+		$committed = $wpdb->query('COMMIT'); // phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
+
+		if (false === $committed) {
+			$error_msg = $wpdb->last_error ?: __('Transaction commit failed.', 'ultimate-multisite');
+			wu_log_add(\WP_Ultimo::LOG_HANDLE, "Installer '{$installer}' commit failed: {$error_msg}", LogLevel::ERROR);
+
+			return new \WP_Error($installer, $error_msg);
+		}
 
 		return $status;
 	}