form-action CSP blocking allowed URL

This is because during login you perform a redirect through the host-source whose is not allowed in the form-action directive (the port, the scheme, domain/subdomain name does not match).
When redirecting, the CSP checks the entire chain of sources, but browsers have differences in the behavior of form-action for redirects:

• Chrome/Safari consider a redirect when submitting a form to be potentially dangerous, since sensitive user data can be redirected to an attacker's domain. Therefore, they block redirection if host-source (domain) not allowed in the form-actions are participate in the chain of redirects.

• Firefox believes that the server redirect is under the control of the owner of the page protected in CSP. Therefore, during redirect it allows you to send the form during redirect even to third-party domains.

Note 1. 'self' means exact scheme://domain:port from the Url in the address bar. Therefore CSP:

 form-action 'self' https: *.mydomain.com

• In case Url is HTTPS://subdomain.mydomain.com the above CSP is become form-action HTTPS://subdomain.mydomain.com https: HTTPS://*.mydomain.com whis is equal to form-action https: - it allows anything except http:-Urls.

• In case Url is HTTP://subdomain.mydomain.com, the above CSP is become form-action HTTP://subdomain.mydomain.com https: HTTP://*.mydomain.com and it does not allow a main domain mydomain.com.

Note 2. The Url https://subdomain.mydomain.com/login/local in the message:

Refused to send form data to 'https://subdomain.mydomain.com/login/local' because it violates ...`

is not Url really blocked by Chrome. This is just the first Url in the redirect chain.

Note 3. If CSP, after all, blocks the allowed domain, it is most likely that it's interference of browser extensions such as NoScript/uBlock/AdBlock/PrivacyBadger, etc. interfere.