TL;DR - is there any specific, one-word, film-specific name for films that are feature-length but don't have a specific ending?
Traditionally, films are said to have three parts- a beginning, a middle, and an end. As an art form, or medium, this is of course sometimes changed purposefully to illicit a response from the viewer. Films which do this will have a feature-length duration, but sometimes will end and cut to credits, leaving the viewer impacted by the sudden close. Alternatively there may be films which end but seem to feel there should be some denouement to wrap up what's been seen, but none exist. These films aren't unfinished or incomplete, just stylised and intentionally dramatic. You might cite films such as Inception, A Serious Man, or even No Country For Old Men.
In contrast to this, there are films which are split into multiple parts, and aren't generally structured as a full, whole film with its own resolution - generally they close to credits with lots of unanswered questions and only journeys half-complete. There are lots of examples of this, and many in the more popular sphere of cinema: Harry Potter, The Hobbit, Twilight, The Hunger Games, The Avengers: Infinity War, and recently even the Fantastic Beasts series. The first part to every two-part film here definitely feels like they have a 'soft' ending where lots of plot lines are unfinished, but almost more feel like there's room for an intermission, but not a sequence of credits as though it should feel complete. More films can be found here: https://en.wikipedia.org/wiki/List_of_films_split_into_multiple_parts
Films as part of a sequence, or trilogies for instance, don't fall prey to the feeling that the plot is unfinished, such as Toy Story, Back To The Future, or even Star Wars, the intergalatic family affair. All films are well-contained and can be considered standalone without consideration for the other films in the sequence.
As an addition, I understand some back-to-back stories and works deserve a 6, 9, 12 hour-long total run-time, and I'm aware some Harry Potter fans would love nothing more than each page of each book be meticulously re-created for the screen and imagined word-for-word. To have the final book split in half for a film-adaptation however, seems simply like a shameless cash-grab when the films cannot stand alone, and to see both halves of one film requires two payments.
TL;DR - Apologies for the ramble, but is there any specific, one-word, film-specific name for films that are feature-length but don't have a specific ending? Having the word 'Part 1' in the title should not excuse poor story construction, as in, the final third of a film not focus on completion of plotlines.