Can leaving out information count as misrepresentation even if I did not actively lie?

Yes. Misrepresentation under immigration law covers withholding material facts just as much as it covers actively false statements — the legal test generally captures both directly or indirectly misrepresenting or withholding a material fact that could induce an error in the administration of the Act. Simply not mentioning something, when it should have been disclosed, can be treated the same as stating something false.

This trips people up because it feels different, morally, from lying outright — many people believe that if they didn't actively write something untrue, they're safe. But an application is generally expected to be complete as well as accurate, and a material omission, such as leaving off a previous refusal, a family member, or a past charge, can be found to be just as much a misrepresentation as an outright false statement, provided the missing information was significant enough to matter to the decision.

Because "material" is doing a lot of work in that test, and reasonable people can disagree about what counts as significant enough to require disclosure, the safer approach is always to disclose more rather than less, and to get advice about anything unclear before submitting an application rather than deciding unilaterally that something didn't need to be mentioned.