[answer copied from another thread]
My temporary workaround was instead of clicking "iCloud" when setting up your secondary emails, to use "Other" then "Add Mail Account" and add it manually. This way you get to type in a description for each account.
Server details and app-specific passwords instructions can be found here:
Mail server settings for iCloud email clients - Apple Support (HK)
______________
[extra info]
Q1: What's the difference?
A1: The account will show in Settings>Mail>Accounts as a "Mail" account rather than "iCloud Mail".
Q2: Are you able to sync Contacts and Calendars as well?
A2: Unfortunately not with this method... just Mail and Notes.
Q3: What happens on your other devices?
A3: Weirdly, on your other iOS/iPadOS devices, the account will be added as the opposite, so it will appear as an "iCloud Mail" account rather than a "Mail" one (very oddly!).
Q4: Can I control the IMAP settings on any of my devices?
Because of the answer to Q3 above, only the device you added the account (the device where is shows as a "Mail" account, rather than an "iCloud Mail" account) gives you access to these (in Settings>Mail (or Notes)>Accounts). You won't have access to these settings on other iOS/iPadOS devices.