To paraphrase an old wisecrack,"At what point in the deer's fatality did the bullet fail?"
The idea of "retained weight" being some sort of weighty (ha ha) metric of projectile performance was not a thing, according to one source, until mentioned by the late great Bob Hagel in his Game Loads and Practical Ballistics for the American Hunter (1978). A good book by the way. Hagel was probably making some correlation to penetration performance, but retained weight in and of itself is a meaningless statistic. If the bullet penetrated sufficiently and destroyed tissue, I wouldn't care of it was left with 0 weight when it was done.
That bullet seems fine for smaller whitetail deer. Having said that, I'm not a fan of 300 grain bullets or Hornady FTX in .45-70 for anything else. I've used their 350 grain FP in the past and had good results up to medium sized wild boars. I've sort of gone full circle and now use 400-425 grain LBT bullets, and accept that the 45-70 is a 100-150 yard rifles under modern conditions.