Google puts random things in Spam, including ones marked 'not spam' or from otherwise reputable senders (like Github, despite receiving their emails for a decade). My monthly credit card bills occasionally are dumped in spam.

Its not really proof of much unless one is constantly getting junked

Back when I used GMail, Google would sometimes mark correspondence e-mails from Google staff as spam. Heck, I even had instances of responses to e-mails I had sent to Google staff marked as spam. Spam filtering is complex and clearly Google has (and continue) to tread a very fine line here.

Last week I saw the same behavior sending email from my work's Google Workspace account to my personal Gmail account. The email stays within Google, I'm logged into both accounts on the same computer, does Google think I am I spamming myself...?

Google's expense reimbursement emails were systematically considered to be SPAM by gmail.

Google knew this because they even issued warnings.

Still, using your own domain and machine is different.

Unless you've had an interaction with that email address it will be considered SPAM.