MacOS – Can ping Mac from machines on network but can’t connect to any of its ports

So, I have been dealing with a rather peculiar issue:

• I have a Mac on my home network connected via Ethernet, which I use for web browsing, checking email, etc. I have no problems accessing both my local network and the outside Internet.
• I have a Linux box and a MacBook also on the same network, the Linux box on Ethernet and the MacBook via WiFi.
• From both the MacBook and the Linux box, I can ping the Mac, and from the Mac, I can ping both the MacBook and the Linux box.
• From the Mac, I can ssh into both the MacBook and the Linux box, but I am unable to ssh from the MacBook or the Linux box into the Mac (connection timeout). Remote Login is enabled in the Sharing preferences pane.
• I can ssh from Mac into itself (ssh localhost, ssh <local_ip>), so the daemon must be up.
• Web servers hosted on the Mac (on port 80, 8000, etc. – I tried several ports just to make sure) are accessible via localhost or local IP on the Mac, but aren't accessible from the MacBook or the Linux box.
• Firewall on the Mac is turned off, and there are no third party firewalls or AV software installed.
• I have rebooted the router, the Mac, the MacBook, and the Linux box many times.
• mDNS resolution works: pinging the mDNS name of the Mac resolves to the local IP.

So, as far as I can tell, it seems that none of my listening ports on the Mac are available to anyone on the LAN, but my outgoing connections still reach them just fine.

Any ideas on what's happening? I'm running out of ideas.