Normally, the shading is controlled by the setting in Tools|Options|View,
and it is worth turning it off to see if that makes a difference.

However, that should only affect the on-screen view (although I believe it
is sometimes erroneously shown in print preview). /Form fields/ may print
with a grey background, but there's no reason you would have those in this
case.

There have been a few reports of this over the years but I don't think
anyone has pinned down a cause. I think someone once mentioned that they had
only seen it when they were using a colour printer with greyscale settings,
or some such. You are welcome to despam my e-mail address (remove
"KillmapS") and send me a copy of something shareable and I'll see what
happens here.