I will admit that this is a guess, but I have to wonder whether it's an issue with the type of film restoration they do when making the disc. With older prints, there can be a lot of dust, scratches, hairs, etc. on the print. Also, depending on a number of factors about how the movie was filmed, it may be grainier than they wanted, but were limited at the time by budget, technology, etc.
To combat this, during the restoration, they'll use a number of filters and processes to reduce the amount of noise, dust, scratches, etc. This can result in a softening of parts of the image. It may be that when text is displayed, the softening is more obvious or objectionable, so they simply turn it off or turn it down in those scenes. I say that because what I see in your examples is an increase in high frequency noise in the whole image when the credits are displayed.
So I asked a guy I know who won a technical Oscar for his work on film restoration systems. He had this to say:
Most likely [at the end credits noise reduction is] turned off, or the company doing it was lazy. Usually when we did high dollar restorations ..., we’d essentially remove the text/credits, clean the image beneath and then clean the text and re-composite them because they have different characteristics (aka in that time they were likely optical effects, so they had double noise/grain and other foreign problems from the compositing process). But we did have several clients that wanted ‘quick and dirty’ restorations that wouldn’t pay for that level of work, they just wanted everything run with the same settings, which obviously wouldn’t work as well in opticals.
What he refers to as "opticals" are when 2 things (in this case text and footage) are combined using a physical process rather than in a computer. For something like credits over footage, they would have to film the credits, then invert them, film the footage with the inverted credits over them, the project that with the original credits on top. All of these steps were done by projecting and refilming, so each step added more film grain. By the time they were done, it was all very grainy.
He added that a lot of times the client was paying by the frame, and they figured nobody cared about the credits, so as soon as the credits started, they'd just stop the restoration process and leave the credits noisy.