Hello @AdamH_work,
Thanks for reaching out to us again. About your question,I understand that you mean this configuration:

I was not able to find any explicit information on the behavior you describe, however, my understanding is that it will simply not boot, to ensure a boot from QSPI you should use BOOTMODE 00 or 01, when FUSE_SEL 1, both will cause a boot from fuses. Please note that setting DIS_SER_BOOT will prevent the chip from booting at all in case of a bad or corrupt image in the QSPI, since you won't be able to download any image using the serial boot.
Let me know if you have any other questions.