Junk folder on iPhone not mapped to spam folder on mailserver

My sister has an iPhone 14 Pro, IOS 18.5.

She has an IMAP email account and recently discovered (although the problem has almost certainly been there for years) that the "Junk" folder in her email account is not syncing with the "Spam" folder on the mailserver.

So if she sends an email to Junk on her iPhone from her inbox, it doesn't sync to the mailserver. But if she does the same on her desktop it does sync to the mailserver.

If she goes to Settings > Apps > Mail > Mail Accounts > then selects her IMAP account > Advanced, she sees the following screen:

As you can see, Junk/Spam isn't listed there - but it should be. The Drafts mailbox on the server is mapped to the Drafts folder on her phone; Sent is mapped to Sent, the Deleted folder on the server is mapped to Trash on her iPhone and Archive is mapped to Archive; but the Spam mailbox on the server should be mapped to the Junk folder on her phone and it isn't listed there at all. All her other folders on her iPhone are syncing fine with the mailserver - it's only her Junk folder that isn't syncing.

Her Junk folders on her desktop and laptop are syncing fine with the Spam folder on the server - it's only her iPhone that has this problem, so it's not a server issue - there must be something wrong with the configuration of her Mail on her phone.

How can I fix this?

Dave