Suspicious PET but negative biopsies?

For the past few weeks my aunt has been going through multiple tests for a large tongue lesion highly suspected to be squamous cell carcinoma that has seemingly metastasized to her lymph nodes. PET scan and MRI show the tumor‘s presence and that it’s invading the midsection of her tongue under the surface. Said they would go the radiation/immunotherapy/chemo route to shrink it instead of surgery.

However, we can’t get a definitive diagnosis because the biopsies keep coming back negative. FNA (5 insertions) in the lymph node and then incisional in the actual tongue, (frozen sample) came back with nothing. Now they want a deeper biopsy sample and will put her under general anesthesia to get it. Is this normal? Has anyone else experienced this discrepancy between scans and biopsies?

Wondering if there’s even a 1%< chance it’s something that mimics cancer instead.