The Format as Currency toolbar button and dropdown list does not follow the default currency set (either of the current default locale or explicitly set under Tools -> Options -> Language Settings -> Languages, Default currency), and additionally sets the fixed locale instead of keeping the original default (system) locale, thus reloading in a different locale will not adapt that locale's separators. Shift+Ctrl+4 and sidebar properties' Currency number format do this correctly, both applying the default currency as set and not altering the cell format's locale.
This was fixed with https://gerrit.libreoffice.org/c/core/+/124025 for 7-2 and https://gerrit.libreoffice.org/c/core/+/124152 for 7-3 and later.