"Dylan may have plagiarised a song".

Not quite in those terms, I think, just pointing out his sources, which can be important. Many performers like to know as much as possible about the songs they sing. Like, for example, the Wild Rover, which, far from being an anonymous song collected in Norfolk (England), started life as a temperence song published in London, with a known author who wrote several similar songs in the 1670's.

(In Lord Franklin, Dylan changed the line about ten thousand pounds to ten thousand dollars, too.)

But I don't criticise him for doing what many others have done, and
I recognise his huge contribution to music (at least in his early days). I still have all his old albums in vinyl. Claiming copyright on traditional music also has a long history, designed by some to ensure that the royalties are not lost to swell the record company's coffers insead.