If you have selected ‘Correct capitalization in name and address fields’ in Constituent data hygiene settings then the auto correct also affects the data for address entered on multiple lines. When you enter an address and use “enter” to start a new line, the capitalization rule causes the “carriage return/new line” to be removed, thereby consolidating the address onto one line. If you have not selected ‘Correct capitalization in name and address fields’ then this does not happens.
It appears that this rule is incorrectly editing out the carriage return/new line as this is not documented anywhere that we can see, and the name of the setting also leads the assumption that this is for capitalization edit only.
This behavior is present in both the Constituent Batch and the Add/Edit Constituent screen.