In that case, annotating the actual type will help bindings use the correct type.
I'm not sure why gplugin_file_source_new was a function before, instead of the constructor it is now, but the latter seems more correct.

Compared the girs before and after:
--- GPlugin-1.0.gir 2025-02-14 02:04:07.065756077 -0500
+++ build/gplugin/GPlugin-1.0.gir   2025-02-14 02:04:25.400858480 -0500
@@ -92,9 +92,9 @@
            line="23">A [iface@GPlugin.Source] that will query plugins on disk.</doc>
       <source-position filename="gplugin/gplugin-file-source.h" line="33"/>
       <implements name="Source"/>
-      <function name="new"
-                c:identifier="gplugin_file_source_new"
-                version="0.39">
+      <constructor name="new"
+                   c:identifier="gplugin_file_source_new"
+                   version="0.39">
         <doc xml:space="preserve"
              filename="gplugin/gplugin-file-source.c"
              line="600">Creates a [iface@GPlugin.Source] that will query plugins on disk using the
@@ -104,7 +104,7 @@
           <doc xml:space="preserve"
                filename="gplugin/gplugin-file-source.c"
                line="607">The new source.</doc>
-          <type name="Source" c:type="GPluginSource*"/>
+          <type name="FileSource" c:type="GPluginSource*"/>
         </return-value>
         <parameters>
           <parameter name="manager" transfer-ownership="none">
@@ -114,7 +114,7 @@
             <type name="Manager" c:type="GPluginManager*"/>
           </parameter>
         </parameters>
-      </function>
+      </constructor>
       <property name="manager"
                 version="0.39"
                 writable="1"
--- GPluginGtk4-1.0.gir 2025-02-14 02:04:13.950794531 -0500
+++ build/gplugin-gtk4/GPluginGtk4-1.0.gir  2025-02-14 02:04:27.138868187 -0500
@@ -107,7 +107,7 @@
           <doc xml:space="preserve"
                filename="gplugin-gtk4/gplugin-gtk-plugin-page.c"
                line="365">The new widget.</doc>
-          <type name="Gtk.Widget" c:type="GtkWidget*"/>
+          <type name="PluginPage" c:type="GtkWidget*"/>
         </return-value>
       </constructor>
       <method name="get_plugin"
@@ -312,7 +312,7 @@
           <doc xml:space="preserve"
                filename="gplugin-gtk4/gplugin-gtk-plugin-row.c"
                line="234">The new widget.</doc>
-          <type name="Gtk.Widget" c:type="GtkWidget*"/>
+          <type name="PluginRow" c:type="GtkWidget*"/>
         </return-value>
       </constructor>
       <method name="get_plugin"
@@ -492,7 +492,7 @@
           <doc xml:space="preserve"
                filename="gplugin-gtk4/gplugin-gtk-plugin-settings-list.c"
                line="635">The new list.</doc>
-          <type name="Gtk.Widget" c:type="GtkWidget*"/>
+          <type name="PluginSettingsList" c:type="GtkWidget*"/>
         </return-value>
       </constructor>
       <method name="get_settings"
@@ -691,7 +691,7 @@
           <doc xml:space="preserve"
                filename="gplugin-gtk4/gplugin-gtk-view.c"
                line="387">The new view.</doc>
-          <type name="Gtk.Widget" c:type="GtkWidget*"/>
+          <type name="View" c:type="GtkWidget*"/>
         </return-value>
       </constructor>
       <method name="get_manager"