Problème
After an update already existing markers are suddenly gone. They don’t show up in the map anymore although they do exist in the dashboard.
Solution
Starting with version 1.3.43 (August 2024) it is now possible to use translations for Locations. If you are using a Translation Plugin please make sure all Locations are correctly assigned to a language. This needs to be done manually once for already existing Locations after upgrading the plugin from a version prior than 1.3.43. New Locations will automatically get assigned to a language.
Ici is an article on how to assign all posts (Locations) to a language in bulk with Polylang. And ici is an article on how to do this with WPML.