@arubislander : Thanks for your advice! However, I did upgrade to 20.04 now with a clean install (wipe). Hence, this late reply. I just couldn't wait to try 🙂
I still have to adjust some things (one of them is Dekko backup and config looks as if restored, it won't send/receive) but I hope and think I will be able to sort it out and it will be workable for me. [edit: Dekko is working after fresh installation and config of the app. I haven't put back config and mails.]

@mschmids : I guess you're using IMAP?
I guess it works then because every change takes place on the mail server.
I didn't explain, but I use POP3, which means I'm downloading and storing my emails on my phone.
The folders are on my phone, and need to be merged there. So your solution may help others, but in my case it won't solve my issue.
Nevertheless thanks for your suggestion!
Just edited my original question to include mail protocol.
Kind regards, Alter

@hummlbach Thank you for the information! A hug!

@mohsen Glad your sorted😊

Yes
It does a similar thing on my meizu Pro 5 although the emails are displayed and can be read after around 45 to 50 seconds has passed