Replica honors its own binlog_row_image value when it sets up read_set.
When a slave thread is applying replicated row events in parallel with the
running online alter, we need all columns to be read from the table
(for the online alter logged row event) not only those that were present in
the pre-image for the replicated row event.

Avoid shrinking the set when online alter is running, i.e. leave it all set