That's just peculiar, George - and I can't find any reference online to this happening to anyone else either. I've ordered many a refurbished Mac in my day and NONE of them have had anything on them - and certainly no NEW Mac has either. Truly bizarre to have it happen once - let alone three times.
Is there a reason you want the iMac Pro over a nicely spec'd iMac? The iMac Pro is really geared to those doing heavy duty processing, and particularly video. (Also good for those doing scientific stuff where
any data corruption is unacceptable). But for most of us, they're overkill - and I say that as someone who does a lot of photo processing, too. Of course, there are a few other nice things about the iMac Pro (more ports, T2 etc.) but overall, I still think I'd probably opt for a regular 2019 27" model if I was looking to replace my 2014 model, which I'm not (yet).
I dunno - if you really want the iMac Pro, and order it and yet another one arrives with peculiarities indicating prior use, I'd push for more of a credit than the £80 and then just reformat the darn thing.
(You did check that all three had different serial numbers, I'm assuming?)