Yes, this is intentional. RFC 6797 states:
The UA MUST replace the URI scheme with "https" [RFC2818], and
if the URI contains an explicit port component of "80", then
the UA MUST convert the port component to be "443", or>>
if the URI contains an explicit port component that is not
equal to "80", the port component value MUST be preserved;
if the URI does not contain an explicit port component, the UA
MUST NOT add one.
NOTE: These steps ensure that the HSTS Policy applies to HTTP
over any TCP port of an HSTS Host.
You should run plain HTTP services on a different domain, or even better, use a HTTP+TLS server as a reverse proxy to the internal plain HTTP service.