If an update set is committed with a DELETE update for a field on the child table and the field on the target instance is defined on the parent table, it is dropped it from the parent table.
Steps to Reproduce
Create a scoped application
Create a standalone scoped table with a column named "number"
Note that scoped fields do not get a 'u_' prefix, so on the database/dictionary/storage alias level, the field name can match an OOB field name
Delete the standalone scoped table
Create the same table again, this time extending Task
Open the application record and publish it to an update set
Note that the update set contains delete updates for the table and the number field
Import the update set on the target instance and preview it
Note that the update set now contains an insert and delete updates for the same table (which will be created in task hierarchy) and a delete update for number field
Commit the update set
Open the task.number dictionary record
Note that the record is broken and no label is shown
Confirm there is no task.number field on lists and forms of task tables
Avoid use of out of box (OOB) field names in scoped applications.
Always check the update set, especially the sys_db_object and sys_dictionary updates as these may trigger schema changes. Special attention should be give to updates with DELETE action.
Related Problem: PRB1253960