DKIM-Signature Header Missing with Gmail "Send mail as"

My DKIM is setup through Cloudflare. The DKIM-Signature header is present when I send emails from roundcube but the DKIM-Signature header is missing when I send emails from Gmail using "Send mail as".

Emails sent by roundcube to Gmail report:

DKIM: 'PASS' with domain [my_domain]
DMARC: 'PASS'

Emails sent by Gmail to Gmail report:

DMARC: 'PASS'

Gmail "Send mail as" setup