keyboard navigation switches off

When I am using the apple find panel on text editing, in particular in TeXShop, the keyboard navigation gets switched off.

After restart of the system, it woks fine at first, but after a few uses, it ceases working, although it is still switched on in the settings menu.

Instead of moving the focus to the next input window, the tab key then selects the text in the current window.

How can I make the keyboard navigation work properly?