The Impact of Edited Videos from Other Apps on Instagram Reach: A Study on Watermarks

Adam Mosseri, the head of Instagram, recently posted a clip explaining a common misconception he has encountered. In essence,

The video posted on Instagram, created from third-party apps, does not affect Reach.
Videos with watermarks from these apps, however, do impact Reach.

Mosseri advises removing these watermarks beforehand to maintain Reach for the video.
Source: Adam Mosseri

TLDR: Remove third-party watermarks from videos to preserve Reach on Instagram.