Standard tests are mixed use, with flash thrown in-
and you should easily be able to get 1000+ shots out of the battery, and significantly more than 1000, not less, if bursting and not using the LCD. This is a DSLR and not a mirrorless camera, after all!
My recommendation: since you need a backup regardless, go get one, and see if it behaves similarly. If it does, the issue may be the camera (itself or your operation of it, by simple elimination). If the new battery behaves as expected, then return/RMA the one that came with the D500. (if worse, return all of it...)
Canon 6D|24-105/4L |70-300/4-5.6 IS USM|50/1.4 USM|50/2.5 CM|50/1.8 STM|85/1.8 USM|Samyang/Bower 14/2.8|24/2.8 IS|Sigma 150-600 | C
Canon EOS-M|11-22 IS|22/2|EF-M 18-55