Windows Graph API having a worse response rate than emails sent through the Outlook Desktop app

Hi there - I've been toying around with Microsoft's Graph API, and it seems like for some reason emails I send through the API just hit spam more and generally get less responses than emails sent through a desktop Outlook client. I've more or less confirmed this hypothesis by A/B testing from the same Outlook account with just a very slightly different subject line.

I am building a custom Outlook client application to improve some workflows specific to the field I work in. The application is configured as a webapp on Microsoft Entra, and as signed-in users use the app, it sends mail on behalf of those users. The requests are coming from a Golang web server hosted on AWS, and I just call the HTTP endpoints for /me/sendMail with the standard library httpClient.

I've setup SPF, DMARC, and DKIM for the sending domains, so in theory shouldn't the emails look the same to a receiving inbox? I am having trouble understanding where this discrepancy is coming from, any ideas?