forwarding emails with attachments - loses attachments

As per the article https://support.apple.com/en-in/guide/mail/mlhlp1017/mac

If you have multiple rules, they’re applied in the order in which they appear in the list of rules; drag a rule up or down to change the order.

Mail stops applying rules to a message when it encounters the Stop Evaluating Rules action, or when a message is transferred to another mailbox by rules used on a mail server or computer or device synced to your computer.

If you use iCloud Drive, your rules are available on your other Mac computers that have iCloud Drive turned on. Files attached to rules, such as sound files or scripts, aren’t available.

Also https://support.apple.com/en-in/guide/icloud/mmc77b56a5cb/icloud

Photos uses your iCloud email account to send photos and videos. Attachments are limited in size to a total of 20 MB per email.

Send you feedback to apple https://www.apple.com/feedback/mail.html

Create a new test user account https://support.apple.com/en-in/HT204443

Configure the mail accounts https://support.apple.com/en-in/guide/mail/mail35803/mac

And then follow the article https://support.apple.com/en-in/guide/mail/mlhlp1010/mac

How large are the attachments? Have you tried going to Mail/Preferences/Accounts/Account Information and checking Send large attachments with Mail Drop?

Mail Drop Limits