I think this is possibly a bug in Webmin.
If you have to add "extra" options into a YAML interface file, like MACADDRESS, if you make a change in the Webmin interface it will re-write the file rather than updating it. So any tweeks you have had to make are gone. I don't think that this happened when using NetworkManager.
It would also be good if more options (like MACADDRESS) were added to the webmin interface to cut down on manual editing. Personally I don't know what all the "important" ones would be, but if the webmin interface mimicked the options found in the typical gui menu on linux probably 99% of what is needed would be covered.
Nigel.