iPad "Grabbing" IMAP Email

I have two iPads and an iPhone 7. I also have a personal domain that includes mail services, so I have several email address @myDomain. All of these email address - and 2 Hotmail/Outlook addresses are mapped to all three devices.

In Settings >> Mail >> Accounts >> {Specific Account},I can see that the visible parameters for my "personal" emails are all set exactly the same - at least as far as I can see.

However, here's the weird thing... Two of my personally hosted email addresses are:-

firstname@myDomain

firstname.lastname@myDomain

I use the firstname-only address for informal chatter with friends and the firstname.lastname account for more formal communication.

But when I send an email to firstname.lastname@myDomain, the older of my 2 iPads "grabs" it - and it never appears on either my newest iPad or my iPhone.

When I go to my ISP and check the mail account settings, as you might expect, all of my accounts are set up the exact same way. In fact, literally the only options I get there are to choose between "email account" and "email forwarder".

When I check the settings for that account on my 3 iDevices, I see that all of them are showing that account as IMAP (the obvious thing to look for; wondered if it had somehow saved as POP3). When I say that it is showing as IMAP in iOS, what I specifically mean is that when I go in

Settings >> Mail >> Accounts >> the suspect account, I get a pop-up window, titled "Account", with fields:-

Name

Email

Description

...

Incoming Mail Server

Host Name

Username

Password

Outgoing Mail Server

SMTP ...

Advanced

At the top of the pop-up window, (above the Account Name entry) is the text "IMAP ACCOUNT INFORMATION".

Obviously I've done something wrong... and the symptoms suggest that somehow this older iPad is grabbing email messages and not allowing the other 2 iDevices to "see" them. But I can't figure out why.

I suspect someone will tell me that the simplest thing to do here will be to delete the Account settings on the suspect iPad, then simply re-enter everything from scratch. Happy to do that. However, although I can't say when this started, this certainly didn't happen when I first added the account. I wanted to preserve the device in case anyone asked me to check anything before I delete the account and try to re-create it, just in case there is something in here that can be verified first.

Any ideas please?

Oh, it probably doesn't matter a whole bunch, but I tend to stagger iOS updates on my devices, so:-

iPad 10.5" - 15.0.1

iPhone - 15.0.2

iPad 12" - 14.8

Thanks in advance for any suggestions.