Reading rows and updating rows on a table, within a single transformation, can cause the database to stop updating, referred to as locking, or slow down processing speeds.
Reading rows and updating rows in the same transformation on the same table should be avoided when possible as it is often causes these issues.
A general solution compatible with all databases is to duplicate the table to be read/updated, then create separate read/update steps. Arrange the steps to be executed sequentially within the transformation, each on a different, yet identical, version of the same table.
Adjusting database row locking parameters or mechanisms will also address this issue.