When I save it, the brackets written as HTML symbols will show correctly in frontend. If I reload the View in backend, the two HTML symbols will automatically change into [ ] and after that the marker in frontend will not show.
UPDATE: If I edit this post, it has the same behaviour.
I have tried to reproduce this on a local test site but could not. I used the same format you used in a map_marker shortcode for the marker_title attribute, and the HTML entity codes for the square brackets were unaffected by any amount of editing or re-saving.
Could you please try to eliminate any possible conflicts by testing with the twentyseventeen theme and by disabling all non-Toolset plugins?
Let me know the results so I can see how to proceed.