Are you on the BTYahoo or the BTMail system. If you are not sure, if you log onto your email account using a web browser and look at the BT logo. If you see Yahoo anywhere it means you are on the BTYahoo email system.
How are you accessing your email on your PC. Is it with a web browser or an email client/app?
On your other phone and other devices, if you are using an email app is it set up as an IMAP account or a POP3 account?
I'm on the new BT mail - the old system was fine. I use a laptop, my phone and a PC to access email, but unless you reload it completely when you start it's easy to forget that you have already filed an email. If you pick that email again - either on its own or with others - the system just displays the rotating circle ('wheel of death') and you are forced to completely reload the page. This doesn't happen on the phone, just with the PC and laptop where I'm working in a google or edge window. On the phone I use an app, mainly, although if I need to search back it works better to use an internet version. Not sure about the settings.... The problem happens also if you access email from two different PCs - it's most annoying if you try to move a batch of emails and just one of them has been deleted or moved on another device, as the whole batch fails rather than the sensible approach: an error message just saying that one couldn't be found whilst completing the action with the rest.
If you are using your email account on two or more devices at the same time, or are logged onto your email account using a browser on two or more devices at the same time, if you change something such as deleting an email on one device and then going to another device you should refresh the inbox by clicking on the circular refresh button on the email menu bar before doing anything.
This will refresh the email account and reflect any changes you made and check for any new emails that may be waiting on the server.
You can go into the settings and set the account to automatically refresh at intervals from 1, 2 or 5 minutes.
Your phone is most likely set up as an IMAP account and as soon as you open the phones app it will automatically synchronise and replicate what has been done on the account when you make changes on your PC and Laptop.