BUG #17187: Incorrect field update operation?

The following bug has been logged on the website:

Bug reference:      17187
Logged by:          Vladimir
Email address:      bugagans@gmail.com
PostgreSQL version: 12.8
Operating system:   Ubuntu 18.04
Description:

Hello. 
Yesterday I've try to mass update one of table in my database and found
strange behavior. Some requests work correct, but some update many another
rows in table. For example, this request was update 77 rows:
UPDATE items SET snmp_oid = 'PLANAR-itm18-MIB::mer.34'  WHERE hostid='10460'
AND snmp_oid = 'PLANAR-itm18-MIB::mer.33';
But key PLANAR-itm18-MIB::mer.33 is unique.
When I've update table by another unique key (itemid), it's work fine.