They are indeed forced to redo all the color timing by going back to the negative. And since they have the opportunity, they go “hey why don’t we just improve the grade”
Leading to a movie that looks completely different.
Somehow this also happened with the French Connection.
By the time True Lies came out home video already existed, so Hollywood had learnt about the long tail of movies in home release.
The kind of movies we don’t have good prints for are usually either obscure, so nobody wanted to pay to keep everything in good condition, or somehow damaged, or like you said, not good films stock or just old enough damage happened.
For its release on Ultra HD, Lightstorm, working with Park Road Post, has built a new 4K Digital Intermediate using recent 4K scans of the original camera negative (confirmed per Lightstorm). This footage has been “optimized” by Park Road’s proprietary deep-learning algorithms. Photochemical grain has been greatly reduced, though not eliminated entirely, while fine detail has been “enhanced” algorithmically. There are also occasional shots throughout the film that appear to have had a bit of old-school noise reduction applied, but it’s hard to be sure if it’s actual DNR or simply just that the Park Road process has been a little too heavy-handed in places.
Unless you're talking about original 35mm negatives (which in some cases might have been lost / destroyed / degraded) where there is normally 4K resolution (with decent film stock - as mentioned before, some of the stock quality from the late 70s and 80s was very bad and digitising it didn't produce the resolution expected), using print copies (one or two removed from the original negative) will generally only give you 2K resolution.
The VFX would have been done at 2K anyway...