Opening a file updates / renders the header and footer parts of Lokalize (menus etc) so I assume the editor is working, but the actual tab page is not rendered at all. The editor itself is not visible, instead the area it would fill is not repainted and keeps whatever was there previously. Probably an issue with the tabs and welcome screen being two separate layers in a QStackedLayout in lokalizemainwindow.cpp and / or the editor tabs being QMainWindows within Lokalize's real QMainWindow, which is already causing redraw issues on at least Wayland on Linux.
This is with at least the most recent version of Lokalize available for Windows, which says its v25.03.70
This was mentioned to me on Matrix already, so setting to confirmed.
I was slightly wrong about rendering- the welcome page remains open and active, with buttons clickable and text adapting to changing window widths. The welcome page is the active welcome page, not a part of the window that hasn't been re-rendered. Meaning the problem likely relates to https://invent.kde.org/sdk/lokalize/-/commit/9337662e8e7b53533fcba2dc88e4a79afc918acd