keybase service certificate errors

My keybase client (macOs Version 1.0.39-20180122031025+5e5798149 (1.0.39-20180122031025+5e5798149) is making calls to the following keybase.io servers over HTTPS, and my firewall is blocking due to an invalid certificate chain:

* mdserver.kbfs.keybase.io
* bserver.kbfs.keybase.io

CURL notes the same issue:
curl -X GET -v https://mdserver.kbfs.keybase.io
* Rebuilt URL to: https://mdserver.kbfs.keybase.io/
* Trying 52.55.102.89...
* Connected to mdserver.kbfs.keybase.io (52.55.102.89) port 443 (#0)
* SSL certificate problem: Invalid certificate chain
* Closing connection 0
curl: (60) SSL certificate problem: Invalid certificate chain
More details here: http://curl.haxx.se/docs/sslcerts.html

Why is Keybase using a certificate NOT signed by a known and trusted certificate authority?

If Keybase wants to self-sign their cert, what ROOT certificate should I import?

Reproduce the certificate failure against mdserver.kbfs.keybase.io and bserver.kbfs.keybase.io with the reported curl command, then inspect the presented HTTPS certificate chain and how the Keybase client reaches these services. Done means identifying whether the chain is misconfigured or intentionally private and documenting the correct trust-root or server-side fix.

Written by the indexing model from the issue text.