PA_PROJECT_ASSETS_ALL lines where ASSET_NUMBER is null

We're using R12.1.3

We run the "PRC: Generate Asset Lines for a Single Project" job - and sometimes, the data created on the PA_PROJECT_ASSETS_ALL table contains lines with the following properties:

ASSET_NUMER = NULL

FA_ASSET_ID = NULL

FA_PERIOD_NAME = NULL

For some of the records, where ASSET_NUMBER = NULL:

CAPITALIZED_COST and CAPITALIZED_CIP_COST are <> 0.

For other records, where ASSET_NUMBER = NULL, capitalized_flag = 'Y' but for most that is set to 'N'.

There are approx 10,000 rows on PA_PROJECT_ASSETS_ALL, and about one third, the asset_number is NULL, and two thirds it is populated.