We have recently seen where some of our customers are adding the following to their SPF records: include:spf.titan.email

That is not our SPF include: statement.

That is a different "Titan":

TITAN | Help Center

If you are using https://cloud-out-c.spamtitan.com for outbound mail, our include: is this:

If you are using SpamTitan Private Cloud, since outbound and inbound mail go through the same server(s), you can add "mx" to your SPF record.

If you have any questions, create a support ticket at:  https://helpdesk.spamtitan.com/