Watchdog behaviour P89V664

The P89V664 has an abnormal watchdog behaviour:
1) Enable watchdog  ->  ok
2) Serve the watchdog  ->  ok
3) Don\'t serve the watchdog:
- Reset condition is detected (5us high on RST @18MHz)
- Device does NOT continue with RESTART user-program (abnormal 
behaviour), and does also not continue the program in the main loop 
(which is expected because the reset condition was given)

Remarks:
- User program works properly on P89C664
- Yes, I know the migration-sheet from NXP
- ALE has pull-up-resistor
- VDD is ok
- Error can be observe every time and on multiple devices

Best regards
Claus