Thanks for posting! Would you mind attaching an example image or two of the pictures that look grainy?
Also, did the pictures look grainy before you exported them? You mentioned they didn’t look bad before you edited them, but often the amount of edits you make will cause the picture to look grainy and thus the graininess could have nothing to do with exporting and everything to do with editing.
If half of the pictures look bad, then it’s probably in the editing (not the exporting) that the problem lies. On the pictures that look grainy, what is the ISO that you were using?
And then, how much did you brighten the images? Brightening images reveals grain a lot, especially in shadow areas. In Lightroom, grain is often revealed most when brightening the picture using the Shadows slider.