I’ve read a lot about this.
I think it is just a legend made up by medieval story tellers. That’s all it is. It has grown up to the proportions of a myth.
Over the ages, the legend of the Holy Grail has pretty clearly been influenced by spiritual and religious ideas from the Celts, Jewish mysticism, Greco-Roman mystery religions, and so forth.
Of course, there is quite a bit of Catholic theology in the legend of the Holy Grail too.
But, all in all, it a mixture of all kinds of ideas, belief systems, and so on.
This is why non-Christians as diverse as Richard Wagner (opera composer), Steven Speilberg (move director), and Dan Brown (novelist) have all made major works based on the Holy Grail legend.
But it really has nothing to do with the Holy Catholic Faith.
I suppose it a matter of opinion as to whether it is harmful to watch movies or read books about the Holy Grail as a form of entertainment. But that’s all it is. Entertainment.
If you want to see a real miracle associated with a cup or vessel, just go to Mass at any Catholic Church.