With multiple instances of the application launcher applet (eg one on each screen, but also happens with two instances on a single panel), it's possible for typed text to go in the search box for the other menu (while it's hidden). It seems that there is always one primary menu that always gets the input even if it isn't the one currently being shown.
STEPS TO REPRODUCE
1. Create two instances of the application menu widget (can be on the same panel or another)
2. Click the first one and type some text
3. Close the first one, open the second one, type some text again
4. Repeat 2 and 3 if the bug doesn't occur right away
Sometimes the text goes in the other hidden menu instance, while the visible one doesn't react at all.
Clicking the other shows that the search box has text in it and a text cursor showing it has focus.
The typed text should go into the currently visible instance of the application menu
Linux/KDE Plasma: ArchLinux
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.12.2
*** This bug has been marked as a duplicate of bug 443131 ***