I do not know how you have selected your markers. You need to click on the string where the markers are located. Every time Wp was updated it writes over directory, the developer now has included the marker Uploads and the ability to add from your media library so your makers are not overwritten. Make sure your custom marker is in the media library and select that file string (not permalink) Under markers and icons you will see the media upload button. Properly saved icons if selected from the pre-loaded SLP icons should look like this, (note uploads and saved icons in the string)
https://cici.storelocatorplus.com/wp-content/uploads/slp/saved-icons/arrow.png
not this (note the old legacy without the word uploads and the word images in the string) This cannot be found.
https://cici.storelocatorplus.com/wp-content/plugins/store-locator-le/images/icons/arrow.png
Bottom line re-select your map marker or if you are using customized markers add to your media library.