The problem seems to be within the “Enable WordPress and third-party widgets?” option I guess. When using the WordPress Sidebar functionality this problem does not occur, so the problem seems to be within the “direct Widget loading” by Tailor perhaps?
At first I thought it was a bug within my custom Widget, but after some further digging it’s also occuring within default Widgets like “Pages”.
When I add a widget directly (so not trough the sidebar functionallity by WordPress), save it and then refresh the Tailor Customizer page, the Widget is no longer visible and therefor editable. Within an tab this also means you can’t click trough the different tabs at all within the customizer and as soon as I publish it again after refreshing, the widget has been dissapeared from the site.
Basicly this means the page has to be tailor fit after one edit, else I’ll loose (widget) data.