GitHub webhook is not sending request with full URL

I’ve set my webhook URL to something like:

http://my.jenkins.domain:1234/github-webhook/

But I haven’t been receiving any webhooks. According to GitHub, the URL is reachable and returns a 200.

When I check the Recent Deliveries, I see that GitHub has been sending webhooks to:

http://my.jenkins.domain:1234

and has been receiving some 200 HTML response. From what I’ve seen online, the request URL that GitHub should’ve hit should have been the full URL (port/path included).

Anyone else run into the same issue?