This is my final word about multiple endings: Maybe.
LotR is all one book, that's how I first read it and that's how I'll always see it. (That happens to be how it was intended, which is convenient but irrelevant.) The second and third* copies I've owned were printed it in three parts, but it's still one book.
For me it's all about expectations. For a series, even a long series, I can handle being left dangling on the intermediate parts, if the ride is good enough that there is no doubt of my heading out for the next one. (Neat endings for each book, like Rowling managed in the Harry Potter books, are best but not absolutely essential.) But the series had better come to a good conclusion - that is a
must!
I remember one series, a trilogy - I thought. A silly, slightly soppy fantasy but good fun. I had so much fun that I happily bought the next trilogy to see how it was going to conclude. ... That wasn't as much fun, and the author started to break their own rules. I was starting to feel disillusioned before I got to the third book of the second trilogy ... and then I discovered the author was publishing a third trilogy - a continuation! It seemed the conclusion was (at least) another three thick books away. Those are three books that have never made it to our shelves, and I've never been able to recommend the first books to anyone because there was no conclusion.
Do
not promise me an ending and then not deliver, or I shall be most seriously displeased.
* Sometimes, it turns out, even good friends cannot be trusted to keep books in good condition. I guess that's one extra advantage of ebooks: either you can't loan them, or if you can they cannot be torn, sat in coffee and all the rest of it.