HTTP proxy not forwarding 301 response

Hi.
We are experiencing a curious issue where a 301 response from a certain website is not passed through the http proxy. If we bypass the proxy things work fine.
I have captured the traffic both on a local pc and on the external interface on the firebox.
Here is the result from the firebox:
GET / HTTP/1.1
Host: www.dfd.dk
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
Accept-Language: da
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Pragma: no-cache
Cache-Control: no-cache

HTTP/1.1 301 Moved Permanently
Location: https://www.dfd.dk/

And here is the result from the local pc:
GET / HTTP/1.1
Host: www.dfd.dk
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
Accept-Language: da
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Pragma: no-cache
Cache-Control: no-cache

This is of course just the textual representation, but i have the pcaps if they are needed.
To verify that it was not an issue with our custom proxy configuration, I have tried using the default http-client.standard settings with the same error.
There are no errors logged. I have also tried enabling debug logging for the proxy, but that does not return any obvious errors either.

The firebox is an M270 running version 12.3.1.B585922

Regards

Jens Hyllegaard

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