Apple Mail overrides my choice of “From” address

My third party email service is accessed via IMAP through the Apple Mail app on my iPad and iPhone. I have several alias addresses on that particular service. Been using it without a hitch for many years. I understand completely how Apple Mail handles different email services and aliases within the same Apple user account. Since installing iOS 14, a problem arose, which may or may not have anything to do with that event. However, it has NEVER happened before on my devices. Whenever I manually select one of the alias addresses and hit send, Apple Mail changes the “From” address to a different alias, the same one every time. It does not matter if I am initiating an email or replying to one, the app will change it to that other alias. It also does not matter if I am passively using the suggested “From” address that appears relating to a given recipient or group of recipients. It will change that too. This causes confusion since I use different aliases for different recipients. This overriding of my chosen alias does not happen when I access my mail service via its online website. It only happens when using the Apple Mail app. My mail settings are correct and have been unchanged for several years. This exact same problem has been reported many times on Apple forums over the last TEN YEARS. I’ve not found any online answers that successfully explain why this is happening or how to fix it. I’ve also not found anything from Apple acknowledging this problem and why it keeps on re-occurring year after year. Anyone have the answer?