[Development] Qt v6.6.0 and v6.6.1 from online installer differences?

[Phil Thompson]

My binary built against Qt v6.6.0 from the online installer fails to run 
against v6.6.1 from the same source. The problem is a missing symbol...

_ZN14QReadWriteLock16destroyRecursiveEP21QReadWriteLockPrivate at Qt_6_PRIVATE_API

...which demangles to 
QReadWriteLock::destroyRecursive(QReadWriteLockPrivate*)

I don't call that directly but I do call the QReadWriteLock dtor. 
Recompiling my code against v6.6.1 solves the problem.

Was this intentional?

Thanks,
Phil