To check to see if it is set up as IMAP, go to Setting>Passwords & Accounts, tap on the Comcast account. On the next page, where Account, and your email address appears, look just above that, and it should have IMAP there. If not, it should have POP. While I empathize with your condition, something about how the same account on 3 different devices is working differently is very strange. My Comcast account works on my Windows computer in Outlook 365, on my iPhone 11 Pro with iOS 13.4.1, my iPad Pro 11 with iPadOS 13.4.1, and on my MacBook Air with Catalina 12.15.4. Everything is perfectly in sync. I don't say this to irritate you or dispute what is happening to you, it is just to say that it can work in sync across multiple Apple devices. Something about your account or how it is configured on devices is causing this issue. By the way, you can check the configuration of both the iPhone and the iPad in the same place on each device to check to see if it is configured as IMAP. What also worries me is that if one is or was configured as POP, that is a completely different server than the IMAP one.