Glitch/pixelation after render

the setup:
Davinci Resolve 17 beta.
Macbook pro (2014)
video shot on iPhone
Render: used youtube default setting at first then custom but still same result

Any idea what this pixelated glitch could be from?
Full video came out great except this one area.
Total video is about 2 minutes with a couple of Text boxes, another short video in timeline, an added audio track (2 of them).

Smartphones are not good for editing, since they produce variable frame rates. Try to convert your originals into a constant frame rate version with Handbrake or Shutter Encoder.

Cell phones aren't good options for filmmaking.

https://www.blackmagicdesign.com/produc ... nemacamera