Qt on MacOS not finding correct (?) LLDB

I have resurrected a C++ project I was working on a few years ago in Qt (MacOS), and can no longer get debugging to work. The problem seems to be that it is trying to use the wrong (old?) version of LLDB. The current macOS version is 12.6.8. I'm not sure what the version was when I set it all up. In Preferences > Kits > Debuggers, it lists as 'Auto-detected' both Applications/Xcode.app/Contents/Developer/usr/bin/lldb, and /usr/bin/lldb. The latter is flagged (popup appears when hovering) as 'Cannot determine debugger type' (and with a red warning marker). The other entry seems fine. When I look in Projects > Build and Run and hover over Build (for this project) the popup window tells me 'Not recognised engine using /usr/bin/lldb', and when I try to run the code in debug mode I get the message (in Application output) 'Unable to create a debugging engine'. It seems that Qt is finding two versions of the debugging engine, one of which is presumably OK and the other not, and then tries to use the wrong one. There is no option in the list of debuggers in Preference > Kits to remove the problematic one or to set a default. How can I figure out these dependencies to get my project to compile?