Saving Illustrator as PDF removes artwork

When saving my Illustator file as a PDF it seems to be removing/not displaying random artwork pieces. On Illustrator it looks exactly how I need it to, as soon as the PDF opens, it's missing content. 

In the screenshots below I have 3 layers.

1. Cutter - Containing the cyan line work

2. Artwork - Containing the purple artwork

3. Dimensions - Containing the red line work and text

It's usually the red line work that disappears on the Dimensions layer but only parts of it vanish. My concern is that if artwork is removed, it might not be picked up on. 

I'm using Illustrator 27.5 (64-bit) and in previous versions, this problem has not occured.