SPF, DKIM and DMARC all passed, and Gmail still sent my domain to spam for weeks

During August I found that emails with my own email domain got into the spam folder in Gmail no matter how I made a delivery. SPF, DKIM and DMARC verification passed smoothly and the domain was not on any blocklist.

I tested four kinds of sending a message with 'test' only and no links: first from my Workspace, then SocketLabs, an old hosting email server, and finally via Gmail API. All four resulted in spam. Consequently, I believed the issue was related to Gmail being against my domain rather than it being the email route that caused the problem.

The situation was dramatically changed when people started replying. I launched a birthday gift sign-up where readers were required to reply to the confirmation mail, and they literally fetched the mail out from their spam folder and replied. Every day I sent a test message to myself, and since September 1st it has landed without problems in the inbox every day.

Has your email domain ever suddenly started going to spam while everything else was okay? How did you manage the problem?