MuTect2 in consistence when treat one sample as tumour and normal

Hi, I have mouse cell lines which have been treated by PGDF. Some of the cell lines turned into tumor, some didn't. When I was using MuTect2 call SNP and Indels for the sample which are not turned into tumor, I tried to treat them as tumor and normal sample. That means, I call variants twice. It turns out that I got zero variants when I treat them as normal sample, but called around 6 to 7K variants when I treat them as tumor sample. And each of them have around 1.2K variants which stated as "alt_allele_in_normal". The cell line comes from inbreed line c57bl/6J. I am wondering why there is zero call when I treat them as normal sample, shouldn't they at least call the 1.2K variants which stated as "alt_allele_in_normal"? Thanks for any useful comments.
Daisy