Couchbase rolling back my update query

We have a bucket having 50k records with one missing attribute. Then we tried executing an update query like below.
select count(*) from my-bucket where orderType is missing; --50k records
update my-bucket set orderType = “MY_ORDER” where orderType is missing; – mutation = 49950

After this, when we again try to find the missing records again and again, the count keeps on increasing.
select count( * ) from my-bucket where orderType is missing; --100 records
select count( * ) from my-bucket where orderType is missing; --200 records
select count( * ) from my-bucket where orderType is missing; --350 records

I am not sure what is happening.

Post explain and what is scan_conssistency.
What is output of your update and mutation count. Did u see any errors in logs.

Yes. I tried many times within 2 hours and it was giving same results. However, after 3-4 hours, it started working successfully. I still don’t know what was the issue. What we think happened was that it occurred due to concurrent modification. There was a background cron job processing records of this bucket. However, i expected an error in the cron job instead of my workbench query. Not sure what happened.