If lab partners form a hypthothesis together and write the same hypothesis worded differently, is that considered plagiarism?
Plagiarism is copying someone else's work and representing it as your own. This is not what you did. The hypothesis is joint work, even if each of you wrote it down individually. The differences in wording are not relevant, it remains joint work.

If any of you would present the hypothesis as their and only their work, this would be misrepresentation of authorship, which is also an ethical misconduct in research. On an ethical "seriousness scale", I would consider it to be about as bad as plagiarism.

To do things correctly, you should note down all contributors when writing down the hypothesis. Using slightly different wording that does not affect the meaning does not change anything, although to avoid confusion I would actually recommend to use the exactly same wording.