Hi Manta LI,

I confirm that the observed behavior is something expected.
Indeed, one of the main differences between the DISTINCT mode and MERGE mode is just that DISTINCT not allow conflation. Ie. the feed does not produce data that replaces the previous one, but each event is distinct from the previous one.
Therefore, if for an update the value of a field is not specified:
- in MERGE mode the previous value is assumed;
- in DISTINCT mode null value is assumed.

You might consider switching to a MERGE mode subscription but with a buffer greater than 1. Please refer to section "3.3.7 ItemEventBuffer in MERGE Mode" of "General Concepts" documentation.
But please you should evaluate that if you switch to MERGE you give up the snapshot with size greater than 1.

Regards,
Giuseppe