Chrome rejects localhost API calls with SSL certificate signed by internal CA Authority

After having Chrome refusing to accept self signed certificates when testing in localhost a web application, following advice from here, here and here I have set up a local Certification Authority and created SSL certificates with Subject Alternative Names. Now I can run the web application in https://localhost and have a green secure icon in the url bar in both Firefox and Chrome.

The problem now is that the application makes API calls to https://localhost:8010. While Firefox handles these requests, Chrome is rejecting them with ERR_SSL_PROTOCOL_ERROR.

Inspecting the certificate returned by the API seems its using the valid certificate and everything is setup fine:

> sudo openssl s_client -CAfile /etc/ssl/certs/ca.pem -connect localhost:8010 | openssl x509 -noout -text 

depth=1 CN = localhost, C = ES, emailAddress = support@xxx.dummy, O = ThermoHuman, OU = Development
verify return:1
depth=0 CN = localhost, ST = Asturias, C = ES, emailAddress = support@xxx.dummy, O = ThermoHuman, OU = Development
verify return:1
Certificate:
Data:
    Version: 3 (0x2)
    Serial Number: 1 (0x1)
Signature Algorithm: sha256WithRSAEncryption
    Issuer: CN=localhost, C=ES/emailAddress=support@xxx.dummy, O=ThermoHuman, OU=Development
    Validity
        Not Before: Oct  9 10:05:14 2018 GMT
        Not After : Oct  6 10:05:14 2028 GMT
    Subject: CN=localhost, ST=Asturias, C=ES/emailAddress=support@xxx.dummy, O=ThermoHuman, OU=Development
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (2048 bit)
            Modulus:
                00:b4:57:e4:13:da:84:cd:53:ca:2e:4d:4b:1d:4e:
                95:54:26:bd:53:b3:75:9f:30:9a:0b:64:74:38:e0:
                7c:c0:88:fd:e7:20:eb:4d:0c:1e:73:fe:1b:3d:79:
                e7:24:88:4f:30:b1:64:ee:d5:45:11:65:cd:8a:e8:
                8f:93:f1:d5:d4:dd:21:f6:12:3b:3f:81:19:79:e8:
                f2:94:ec:e1:d5:11:42:f0:71:99:75:6a:a5:44:50:
                5a:9d:56:93:b7:d2:10:b9:22:9e:e4:ab:2b:5d:be:
                1c:a5:1b:5f:34:d2:31:a0:9e:ab:bb:02:23:6c:21:
                fc:9e:f3:c0:7e:ad:0a:2f:cb:23:a7:0c:02:68:1f:
                92:b0:51:de:39:7d:d9:ee:e8:13:f2:2f:99:b9:60:
                c0:d4:77:65:e5:67:38:4e:c0:87:1f:07:de:1d:bf:
                f4:63:84:ba:f3:04:4d:2e:52:49:6e:0a:f5:2c:62:
                4f:22:ae:42:d0:6a:e6:9f:07:bf:34:7e:2c:45:b0:
                a3:06:37:51:94:93:14:06:2a:3c:84:ef:9b:73:39:
                33:5b:31:a0:db:09:99:38:8e:08:68:dd:b9:2e:7d:
                e7:bf:34:a5:d2:47:80:0e:ee:89:75:d5:cc:b4:c1:
                f3:3a:bd:f7:b6:18:98:1a:f0:ac:bf:0a:52:30:54:
                a7:21
            Exponent: 65537 (0x10001)
    X509v3 extensions:
        X509v3 Basic Constraints: 
            CA:FALSE
        X509v3 Key Usage: 
            Digital Signature, Non Repudiation, Key Encipherment
        X509v3 Subject Alternative Name: 
            DNS:localhost, IP Address:192.168.0.254, IP Address:127.0.0.1
Signature Algorithm: sha256WithRSAEncryption
     38:f3:61:1a:c3:6c:93:14:39:d5:ad:a8:7d:62:08:cc:c5:8d:
     f6:45:09:39:d5:8f:e2:a9:ac:16:f9:eb:d8:8d:b3:6a:07:3a:
     97:26:3d:cf:5a:1c:43:ae:c5:e7:7b:44:33:a2:68:be:e4:a6:
     ba:90:c2:cf:5a:d4:55:d7:e1:2e:1c:d0:9b:85:77:e6:5c:a0:
     0e:49:fa:80:07:4d:22:eb:93:3f:28:2b:02:af:35:0c:2a:b6:
     20:7b:09:bd:8b:4b:3b:26:f1:bc:cc:14:e3:09:88:40:89:56:
     bb:1b:ae:12:bf:8f:d7:74:10:7b:30:25:08:de:37:70:67:a2:
     a3:23:cc:a9:ea:c5:1d:08:76:fd:a4:5f:4c:53:5b:42:b2:81:
     26:3b:c6:c7:48:cf:ea:c7:21:92:02:45:33:f8:02:9d:a0:b0:
     d2:a1:48:3f:ed:c9:66:69:e4:49:b4:bb:7c:30:e7:3a:3e:dd:
     92:33:1d:a8:1d:38:89:75:ae:90:92:86:ef:ac:3f:0e:21:15:
     57:09:19:58:20:84:94:59:3c:d1:09:33:2e:74:dd:26:cd:81:
     a2:bb:e7:d1:5e:af:99:dc:68:da:cb:bb:c6:b6:f3:50:9e:80:
     db:8a:31:4f:1c:f8:51:90:d8:ad:21:d0:0c:3f:16:3d:59:0f:
     6e:ed:69:17

Just to test I have put a test.html file in the API endpoint so I can make a normal HTTP request and see what happens.

In Firefox I get for both pages https://localhost:8000 (the web app) and https://localhost:8010/test.html (the API endpoint) a green lock in the url bar. The certificate is correct and the connection is encrypted using TLS 1.2 in both requests.

Doing the same in Chrome fails for the API endpoint. The Security Tab of Developer Tools reveals a Certificate Valid and Trusted but it does not have the Connection Secure with TLS 1.2 indication. See the screenshots below.

This is the Web App request in Chrome: This is the Web App request in Chrome

This is the API endpoint request in Chrome: This is the API endpoint request in Chrome

I don't know where to look for the error so, how can I debug what is happening in Chrome?