Mail Sending From Wrong Account

I have two emails set up and signed in on the Apple Mail app on my Mac: an iCloud account and a Gmail account. I have noticed an inconsistent and very annoying issue with Mail regarding these two accounts (a previous user on here said they've had this same issue since 2012).

If I receive an email on my iCloud, Mail will sometimes autofill my Gmail as the reply account. This means that if I don't go in and manually change the "From:" section to my iCloud address, my reply will be sent from my Gmail with my iCloud CC'd (my reply to a message originally sent to my iCloud!). This happens probably 50% of the time.

It is incredibly frustrating to accidentally reply to an email with the wrong account. Aside from manually checking the "From:" section every time I send a reply email, how can I stop this from happening? For reference, I am currently running macOS Tahoe 26.0.1 (the latest Mac software at the time of writing).