120 fps footage in a 30fps sequence results in choppy footage

Hello,

I want to start off by saying I make a lot of shortform content like this all the time and I never ran into an issue with 120fps slomo in a 30fps sequence before today . I was having no issues with this edit until I rendered and exported. Once I exported the video it was super choppy and everything I put into slomo after now results in choppy footage no matter the sequence settings.

Any help would be great, this is a paid project that usually results in a quick turn around but I have met a wall I do not know how to climb.