In reply to Francesco V..

Sound like this issue https://core.trac.wordpress.org/ticket/48623

The existing links, aren't broken, but when you try to generate a link for some of existing posts it comes out wrong (which in turn might affects canonical tags and so on).

This is caused by caused by date_default_timezone_set() calls to change default PHP timezone setting an a theme/plugin, which is something WordPress core doesn't support.

Counter-intuitively this was caused by work on preventing changing PHP timezone from breaking things, it's just that improving it in one place made it break in adjacent one.

In a nutshell - core still works fine under normal circumstances, but if you already had the issue then that broke things in different way in 5.3.

This happened because core it unit tested under "normal" configuration and the issue is rare enough and affects few enough post that it was never reported during development and pre-release versions.

There will be a unit test for generating permalinks breaking in this way now and there is proposal to check for this in Site Health tool.