Hi,
Am not sure about where exactly the data of APO Temp order no and ECC original order number saved in ECC.
Please check the following.
In APO IMG for global settings for number ranges of orders, check PPDS > Global settings > maintain no range of orders.
IMG> APO > supply planning > snp > basic setting > configure to oltp setting.
You can give a try on this table CIFORDMAP in ECC, which may have the GUID of your PO, Though am not sure how you'll able to find the relationship with temp & original order.
----
If all the settings are correct.
Make note of your current/latest planned order.
Make sure no other planning jobs are running or executed manually across the system.
Try creating a new planned order and check the order number whether its continued with your previous number.
Just my thoughts
Regards
Vinoth