Glue records have nothing to do with this situation so let’s stop that conversation here. @BeardedRazorback
First off, do not proxy email through CloudFlare. I am honestly shocked that it works. Now, on to your concern…
You need to tell your web form which address to use as the sender address, and this address must be either the address that you use to authenticate with SMTP to your MiaB (something that you are not doing using PHPmail) or a permitted sender of that address. Note, since it plays a role in domain validation the use of admin@ email addresses is not advised.
So as you have described it, this is not a MiaB issue at all since you are sending emails directly from the server using PHP. But if you want to send via MiaB using SMTP you need to take heed of the requirements I have listed.