Contact merge is not working according to defined unique identifier field

System Information
Mautic version: 3.2.0
PHP version: 7.2

Mautic provides contact merging feature Contact duplicates

According to documentation I have set one custom field (c_hashkey) as unique identifier. I am creating/updating contacts via API and they do have unique c_hashkey. Still they are merged and keep updated with reference of same contact.

As shown in above screenshot, c_haskey is unique everytime but the same contact is getting updated everytime.

Also there is not any other field which is same to the old data, Still it it getting merged repeatedly.

Can anyone explain on what basis the contacts are merged and why it is still happening even after providing unique data for unique identifier(c_hashkey)?