Off By One in POST port numbers....?

This might be off topic but I don't know where else to put it.

Story time.

I'm setting up a new hub to replace my old one. I had the homebridge/makerAPI plugin running for years on the C5. In prep for the change I added the tonesto variant of the homebridge to the C5 and put homebridge/makerAPI onto the C7. Then both tonesto on C5 and makerAPi on C7 started demonstrating this bug.

The system which both makerAPI and tonesto use to setup their POST events isn't working properly. They both attempt to configure a POST back url, but an incorrect port number is saved. It's always off by one -- suspicious.

I turned on all logging to find this problem, and I see this:

Homebridge Logs:

[11/29/2021, 11:42:12 PM] [Hubitat (C7-MakerAPI) hhm:0.4.15] Setting MakerAPI post URL to at
[11/29/2021, 11:42:12 PM] [Hubitat (C7-MakerAPI) hhm:0.4.15] homebridge-hubitat-makerapi server listening on 20011
[11/29/2021, 11:42:12 PM] [Hubitat (C7-MakerAPI) hhm:0.4.15] MakerAPI postURL successfully activated

Hubitat MakerAPI Instance Logs:

app:152021-11-30 01:52:54.197 am debug url=
app:152021-11-30 01:52:54.194 am debug setPostURL called

The MakerAPI instance is setting the Post URL incorrectly, off by one. In this case the homebridge plugin started it's server on 20011 and asked the server to set the post URL to 20010. What I can't understand is where this could be going wrong.

When I manually correct the port number in the MakerAPI instance settings, I'm able to send events back to homebridge and see the logs on both ends. Unfortunately danT's MakerAPI homebridge plugin frequently re-updates the POST URL so this fix only lasts a handful of seconds.

I saw similar behavior on tonesto's homebridge setup and it was diagnosed as "the hubitat app cannot connect back to the homebridge plugin", which is consistent with what I'm seeing with makerAPI.

The ancient mac mini that's running homebridge is also running a node-red instance and that instance is able to communicate with makerAPI and configure itself to receive events back from hubitat without issue. This seems to rule out firewalls somewhere in my network, not that incorrect port numbers could even be influenced by a firewall.

It's a bit of a weird problem and I'm kind of at a loss about it, any suggestions or advice are appreciated.