From iOS Doc https://developer.apple.com/documentation/corelocation/cllocation/1423550-verticalaccuracy

verticalAccuracy When this property contains 0 or a positive number, the value in the altitude property is plus or minus the specified number of meters. When this property contains a negative number, the value in the altitude property is invalid.

However I don't see the code accounting for this:

Instead a check for negative vertical accuracy, then set altitude to null.

This could mean a user could get an incorrect altitude, when altitude isn't available.