Odd timestamp applied in update when timestamp variable in stored procedure is null 0000-00-00 00:04:00

Database
Enthusiast

Odd timestamp applied in update when timestamp variable in stored procedure is null 0000-00-00 00:04:00

In a stored procedure a timestamp variable has been set (via a cursor select) to null.

When using that variable to update a timestamp column of another table, the updated value resolves as

0000-00-00 00:04:00  instead of null.

It does not vary when many records are processed through the SP, always just 4 minutes. 

Have not been able to predicted when we will get a null (as desired) or get a 0000-00-00 00:04:00 (not)

No date math occurring nearby. 

  update x_tbls.aircraft_custody

  set end_ts = c_custody_end_ts,

  source_name = 'XXXX', last_alter_ts = w_last_alter_ts where

  acft_serno = w_acft_serno and action_ts =

    (select max(action_ts) from x_tbls.aircraft_custody

     where acft_serno = w_acft_serno and

     action_ts < w_action_ts);