I tried to move left the placemaker and it looks like better now! (apart for the old label which is still half visible, should be a temporary issue?)
First I decided to change the place with locality as suggested but as i realized that in the nearby there are many others similar places already marked as isolated_dwelling (they are mostly abandoned huts used for mining in the past) the labels have been rendered in different styles, so i prefer to let them all similar to make the map better readable for hikers on that trail.
thanks to the prompt help in the IRC chat with some guys i’ve resolved the issue;
I simply did the /dirty trick on the tile that hasn’t been updated for every zoom level (16 - 19).
For people just like me who didn’t know that trick, simply get the selected png tile by using the dev tools (press F12 in the browser), then go to the NET panel and discover the tile; then call that url in a new window, adding /dirty at the end of the url: the server will respond with the message that the tile will be update (a few minutes normally). Also the /status request will show you some information about the last rendered and access time.