Emails disappearing from imap email account on Priv, despite still being on server
I've recently upgraded to a Blackberry Priv and have three email accounts under my business domain name, each of which have Home Screen shortcuts set up on my Priv, as well as appearing collectively in my Hub.
One of those email accounts is dedicated to incoming ORDER notification and TRANSACTION notification emails so the only emails that come into this account are a) ORDER notifications that are generated whenever an order is placed on my website and b) the TRANSACTION notifications from my merchant payment processing company. On my Priv and with my email host, I have these accounts set up as imap accounts and they're all set up for emails to stay on the server forever. All the settings on the three accounts have been set up in an identical way on the Priv, with exactly the same settings.
However, I've noticed that the email account for the order notifications only shows the latest ORDER notification email and then as soon as a new ORDER email comes in, the previous one(s) disappear (from the individual email account and the Hub). However, the TRANSACTION notification emails all continue to show in the Inbox for that email account on my Priv and both the ORDER and the TRANSACTION emails still show in my Inbox when I look at this account via my Webmail login, which ought to be mirrored on my Priv's email account. I've not noticed this happening with either of the other two email accounts I have set up on my Priv and can't understand where the ORDER emails are going - nor why they're behaving differently to the TRANSACTION notifications in that email account and why it's only in this email address's account that emails from a particular sender are disappearing on my Priv. (The most recent ORDER notification doesn't disappear until a new ORDER notification email from my website comes in.)
I've only just noticed this and don't get orders through very often but I do sometimes need to refer back to a historic order's details when I'm out and about and the easiest way to do this is to check the ORDER notification email on that email account.... but of course, I can only see the latest order.
I've repeatedly tried deleting the email account on my Priv and re-adding it (Restarting the phone before re-adding it). I've checked the few sub-folders I have for this email account and the vanishing emails aren't being forwarded into those. I've made absolutely certain all the settings on the three email accounts are identical and the missing emails are definitely all still where I'd expect them to be, in my Inbox of my Webmail account for this email address.
I'm so sorry for the lengthy explanation but does anyone have any ideas why this email account only on my Priv isn't reflecting what's on my email host's server?
I'd ideally prefer to not have a work-around or to set this email account up via a Gmail account. I upgraded from a Blackberry Classic (and have had many Blackberry devices before that) but have never had a problem before with this email account. I'm not sure if this problem has just recently started as I've only just noticed it, or whether it has been happening since I had the phone. My business has gone very quiet recently so I don't have a regular flurry of orders to help observe this frequently.
(My techie knowledge is quite basic, so please can any answers be in the "...For Dummies" level of language!) :)
Many, many, thanks, to anyone who can point me in the right direction! :)