This problem has been around for a long time, I've reported previously, maybe not well enough.
So after doing it a few times, with a few months in between having to do it again and forgetting what I did last time, my new method was quite easy.
Use 7zip, edit the metadata.json file in the section for the trb1451-modem.ubi the sha1 is incorrect, so overwrite it with "3cde6af4c7d2c766c25e5fb77bd9d840339bff16" and save the file and it update the archive file.
Also this is the sha1 calculated by 7zip too.
I am assuming that the file is correct, because it is the same file in legacy bin files for the trb1411 & trb1421, and if it ain't updating it again with v2.06.1 afterwards anyway. trb1401 has a different modem file.