Could not serialize access due to read/write dependencies among transactions

Hi,
we have a Spring Boot application that tries to update two separate rows in
a table in 2 separate threads, and it uses the default Transaction
Isolation level of READ_COMMITTED.
One of the two threads always succeeds, the other always fails with:

org.postgresql.util.PSQLException: ERROR: could not serialize access
due to read/write dependencies among transactions
[java] ERROR> Detail: Reason code: Canceled on identification as a
pivot, during write.
[java] ERROR> Hint: The transaction might succeed if retried.

I thought that error could only happen for SERIALIZABLE transactions, and
moreover, the two threads are trying to update different rows.

The sql update has a where clause on the primary key:
update table set status='a' where column_a = {value1} and column_b={value2}

The table's primary key is a composition of column_a and column_b

thread1 and thread2 use different values for value1 and value2

Does anyone have any idea how the problem could happen?

Thanks,
Michele