Usually the CY2308 is not designed as a Fail Safe device. However, the PLL of the 2308 will not lose lock as a consequence of the oscillator being pulled by a few hundred PPM. The 2308 PLL will be able to track any low frequency variations in the reference input and will filter out any high frequency variations greater than the loop bandwidth which is around 1 to 2 MHz, typically 1.5 MHz.
Cypress System Engineers have tested this glitch operation for different reference frequencies: 33MHz, 66 MHz, 100 MHz, and 133MHz. The signal at the PLL output remained for the minimum of 2.3uSec after the input signal was shut off. This free running output frequency drifted slowly but no glitch was observed in any case. The measurement was under 3.3V power supply room temperature. So accordingly, it would not lose lock with a single glitch.