Interesting. It's been a while since I've looked at this code, so will double check my migration files. I'd be running a refresh on the plugin though, assuming you haven't gotten too far into development, to check that it isn't a buggy install.
Same happens on RC 210 with clean installation, seems something changed in FormController handling field configs (as it now tries to save data to model based on config, not present POST data (your modifyPost method works) - this can be fixed with adding Purgeable trait to your Menu model and by adding those fields to $purgeable array.
I have the same problem. I added columns called "internal_link" and "external_link" into the database, with datatype VARCHAR (255) and the saving works. But how can I get the full menu list in the frontend? Is there any function? Is there any
menuManager::renderMenu() ? Which gives these output:?
At some point in the last 6 months (I've had other priorities, sorry), OctoberCMS has changed the way information is saved that broke my logic. I've added fixes for this now, so if you are running 1.4.0 you should not have any issues with saving menu items anymore.