1. When other signals near in frequency coincide with a faded signal, the faded path has suffered from what is called a "frequency selective fade." With the irregular nature of the ionosphere, this can be rapid and lasting, especially when a band is about to drop out entirely, as many have experienced. It is usually an entire band of frequencies that fades when the MUF changes.
2. Non-faded signals on or near the faded frequency, being from diverse locations, have taken other paths via the ionosphere. They are subject to other simultaneously existing ionospheric conditions affecting those paths.
3. Frequency selective fading can be enhanced by simultaneous other forms of fading that may not be noticed prior to the dropout. The following link, although discussing forms of fading relating to equatorial paths, discusses some of those:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001RS002495 4. Frequency selective fading of critical paths can only be counteracted by a frequency diversion system, simultaneous use of multiple frequencies. Non-critical paths suffering from prolonged fading must continue communication on a predetermined alternate frequency.
5. Changing polarization or using polarization diversity would not have much effect on frequency selective fading. It would have an effect on certain other types of fading and, of course, polarization loss in the absence of profound frequency selective fading.
6. Having a waterfall or spectrum display to view after signal loss discounts a strong continuous carrier overloading the receiver. Such a signal causes loss of the received signal, but the noise floor usually disappears, too.
7. Any solar or geomagnetic disturbance would affect a range of frequencies instead of just one.
8. However, rapid, profound, and lengthy signal loss during a QSO in or near Roswell, NM raises the intriguing possibility of other causes!
Mike WBØDZX