This is with Debian's gobject-introspection 1.36.0-2+b1.

When I scanned the structs below and implemented the dup_parameters_array() vfunc, pygi and gjs wrongly placed the pointer in the identify_account() field - that's 56 bytes too early. That error would be consistent with g-i thinking the TpDBusPropertiesMixinClass (64 bytes) was a TpDBusPropertiesMixinClass* (8 bytes) when it calculated the offsets of the vfuncs.

-----------------------------

Faulty structs:

typedef struct _TpDBusPropertiesMixinClass TpDBusPropertiesMixinClass;

struct _TpDBusPropertiesMixinClass {
    TpDBusPropertiesMixinIfaceImpl *interfaces;
    /*<private>*/
    gpointer _1;
    gpointer _2;
    gpointer _3;
    gpointer _4;
    gpointer _5;
    gpointer _6;
    gpointer _7;
};

struct _TpBaseProtocolClass
{
  GObjectClass parent_class;

  TpDBusPropertiesMixinClass dbus_properties_class;

  gboolean is_stub;
  const TpCMParamSpec *(*get_parameters) (TpBaseProtocol *self);
  TpBaseConnection *(*new_connection) (TpBaseProtocol *self,
      GHashTable *asv,
      GError **error);

  gchar *(*normalize_contact) (TpBaseProtocol *self,
      const gchar *contact,
      GError **error);
  gchar *(*identify_account) (TpBaseProtocol *self,
      GHashTable *asv,
      GError **error);

  /*<private>*/
  GStrv (*_TP_SEAL (get_interfaces)) (TpBaseProtocol *self);
  /*<public>*/

  void (*get_connection_details) (TpBaseProtocol *self,
      GStrv *connection_interfaces,
      GType **channel_manager_types,
      gchar **icon_name,
      gchar **english_name,
      gchar **vcard_field);

  const TpPresenceStatusSpec * (*get_statuses) (TpBaseProtocol *self);

  void (*get_avatar_details) (TpBaseProtocol *self,
      GStrv *supported_mime_types,
      guint *min_height,
      guint *min_width,
      guint *rec_height,
      guint *rec_width,
      guint *max_height,
      guint *max_width,
      guint *max_bytes);

  GStrv (*dup_authentication_types) (TpBaseProtocol *self);

  GPtrArray * (*get_interfaces_array) (TpBaseProtocol *self);

  GPtrArray *(*dup_parameters_array) (TpBaseProtocol *self);

  /*<private>*/
  GCallback padding[3];
  TpBaseProtocolClassPrivate *priv;
};

-----------------------------

Workaround:

#ifndef __GI_SCANNER__
#ifndef __GTK_DOC_IGNORE__
/* See below. If this assertion fails on your platform, you'll have to
 * do something clever. */
G_STATIC_ASSERT (sizeof (gpointer[8]) == sizeof (TpDBusPropertiesMixinClass));
#endif
#endif

struct _TpBaseProtocolClass
{
  GObjectClass parent_class;

#ifdef __GI_SCANNER__
#ifndef __GTK_DOC_IGNORE__
  /* g-i gets confused and thinks TpDBusPropertiesMixinClass is only
   * one pointer long (as if it was a TpDBusPropertiesMixinClass *),
   * resulting in everything after this point being off-by-56 on x86-64.
   * Trick it into choosing the right offset - it does understand
   * arrays, so we can use one of those. */
  gpointer dbus_properties_class[8];
#endif
#else
  TpDBusPropertiesMixinClass dbus_properties_class;
#endif
...