The problem might be that someone bought the rifle and never got a chance to use it since they screwed up the holes and never got a scope on it. Thus it appears new, but isn’t.
But the problem could also be with Remington.
FWIW, I bought a new Remington LTR maybe 5 years ago. It had plug screws in the receiver holes, but one screw had half of the top part snapped off along the slot. Using a hammer and punch I was able with much difficulty to get the screw unscrewed (and wound up scratching the receiver in the process!).
I then saw the problem! The hole was only threaded half-way. Somebody at the factory had cranked the screw in, snapped it in the process, and just left it as it was. I also found one of the other screw holes was only partly threaded. Luckily, I had a 6X48 tap and was able to clean the two holes up.
Anyway, converting to 8X40 is an acceptable solution to your problem (and is actually an improvement). OTOH, either you’d need 8X40 mounts or you’d need to convert the mounts you have to 8X40 screws.
Unfortunately, the forward hole in the front receiver ring bottoms out on the barrel. To tap it the barrel would need to be removed.
It occurs to me that maybe Remington has started using 8X40 screws and maybe that’s what you’ve got. Still, the absence of plug screws really suggests someone has fiddled with the rifle.
Hope things turn out OK!!
ETA: Did you get a warrantee card with the rifle?