Hi @robertpetrovics,
To answer your question directly - when updating or removing the User ID (column name: user_name), it is not OOB behaviour to replace this with the sys_id.
However, this is based on a OOB configuration where the User ID field is not mandatory, meaning it can be empty.
I appreciate on the screen shot you've provided, it is also not mandatory, but I would suggest this update is caused either by the AD integration, or plugin, or a Business Rule on the User table (table name: sys_user) which is setting this.
As @SN_Learn has suggested, to find out a little more and help you determine exactly where this maybe happening or what user (system or AD integration user for example), chec the 'history' of the record. This is possible if the user table is audited (Which is not set by default OOB)
To view the history, simply open the user record as provided, and in the grey bar at the top of the form, right click and view the history
Also - tip, when sharing images, I would remove any PII (Personal Identifiable Information such as names and email addresses).
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Thanks, Robbie
