Field backgrounds showing on flattened pdf when viewed in a browser

Hi folks, suspect there's not an answer to this one but if anyone can suggest anything I'd appreciate it.

I've got a pdf document with fields in it so that some of the info can be changed by the end user. The pdf is flattened and locked then uploaded to an online archive. The problem, which is barely visible in below screenshot, is that the fields are still showing up with a slight tint in them. It looks like it has 'highlight existing fields' on but as pdf is flattened there are no fields.

Any ideas?