Use of router MAC address by ISP

I run one node in a network where the computers communicate via IP
address. The software used is proprietary, doesn't use URLs, and
doesn't have the ability to sense when a connection has dropped out.
(The protocol used is UDP.) So, it makes life much easier if I have a
relatively constant IP address.

I have a router, and a backup router, and in the setup for both I
designate the MAC address of still former router. My ISP doesn't really
know should I swap routers -- and I seem to draw the same IP for long
periods of time. But, the IP address they give me is considered
dynamic, not static, so it is subject to change.

Should I hang some other device on the DSL modem (like a computer), it
will draw a different IP, but when I put the router back on line, it
will still pick up the IP it had been using. I assume if there was a
long period, where the lease had expired, that might not be true.

The present router is now almost a year old, and from the beginning I
have been in constant communication with the manufacturer about a
software problem with it. They have supplied several firmware updates,
none of which fixed the problem I was having until just now. But, I
have seen other things changed and generally improved over the period of
time. Finally, the problem I was having has been fixed.

However, with this latest firmware update, another quirk popped up. It
no longer seems to recognize that I am trying to specify a MAC address
different from what the router was assigned when it was manufactured.
At least, when I installed the latest firmware, I was issued a new IP by
my ISP, and it doesn't make any difference whether I tell the router to
report out a MAC I specify, or to use the one the router has.

I complained to the router manufacturer about this, suggesting that
possibly they had a new bug in the firmware. (At this point, since we
have had to reconfigure the network nodes for the new IP, and I am now
running with the default MAC in the router, even if they were to fix it,
I wouldn't change things here now.)

The response I received was the ability to specify the MAC address has
no effect on the IP address that I receive, and that ability is provided
in the router only for where a ISP is using a MAC address for
validation. Is that true? I know in my own LAN I use DHCP but reserve
IP addresses for specific computers based on MAC.

Did the firmware update contain something buggy, or did my ISP decide it
was time to give me a new IP at exactly the same time?

....Bob