good thing i wasn't the only one who noticed that, too.
i kinda noticed that there are a lot of similarities in the HP literature that i think is inspired by Tolkien's saga.
like the names, for instance...Aragorn and Aragog (spider's name), Grima Wormtongue (Saruman's minion) and Wormtail (Lord Voldemort's follower) to name a few.
then the dementors...the first time i saw them, esp the claws/fingers, i kinda remembered the Ringwraiths...
i guess the only difference is: Ringwraiths are downright evil. Dementors don't know the difference between enemy and...well, non-enemy.
Pure ignorance, they are both derived from the Grim Reaper image which is very much overused in Western folklore. There is not a single original piece of work done in art or literature - everything is by necessity a copy of stuff which has already been done as that is what the artist will have been influenced by.
And for the record JKR didn't even like LOTR, but prefered the Hobbit 😱