Some default directories are always write protected others always writable
In settings/miscellaneous/default directories selecting mapsOnline, mapsVector, backup and data/srtm directory starts a system file browser and any folder on external SD is OK and writable. Vector maps from LoStore and map styles from openadro install without problems in masVector folder.
However selecting mapsOnline, maps, mapitems, export and data/geocaching starts a Locus file browser and every folder on external SD is write protected.
I'm using Sony Xperia XZ1 Compact with Android 8 and Locus Map Pro 3.35.2