Just a follow up, in case anyone is interested. POSIX/SUS specified nothing at the time of the original question, but in the POSIX.1-2016 edition (i.e., POSIX.1-2008 + Technical Corrigendum 2), the following text was added:

If a thread has deferred cancellation enabled, a signal-catching function is called in that thread during execution of a function that is not async-cancel-safe, and the signal-catching function calls any function that is a cancellation point while a cancellation is pending for the thread, the behavior is undefined.

This was added after this bug report (cloned from an earlier bug report) by the OP.