Hello,
We are currently investigating this and you can follow this bug for updates:
https://bugs.chromium.org/p/chromium/issues/detail?id=1158169
If your server redirects the form submission to an insecure http:// URL, that may be the cause of the warning, so you might want to check that the entire request including any redirects is sent over https://.
Some features in Chrome are turned on gradually, so not all users may see them at the same time; that might explain why some users are seeing this warning and others aren't.
Emily