I ran composer require 'drupal/fullcalendar_view:^5.1' and the upgrade appeared successful (and I cleared the cache). When I go back into Drupal, it says I'm still on 5.1.11. I tried running the composer command again and it says there is nothing to install. I've successfully updated this module many times before.
Update: I just tried upgrading it on a second website and the ^5.1 command returns there's nothing to update. It probably did that on the other website too, I was just going so fast I didn't notice.
composer require 'drupal/fullcalendar_view:^5.1'composer require 'drupal/fullcalendar_view:^5.1.12'Root composer.json requires drupal/fullcalendar_view 5.1.12, found drupal/fullcalendar_view[dev-1.x, dev-2.x, dev-3.x, dev-5.x, dev-6.x, 1.0.0-alpha1, ..., 1.x-dev (alias of dev-1.x), 2.0.0-rc1, ..., 2.x-dev (alias of dev-2.x), 3.x-dev (alias of dev-3.x), 4.0.0, ..., 4.5.0, 5.0.0, ..., 5.x-dev (alias of dev-5.x), 6.x-dev (alias of dev-6.x)] but it does not match the constraint.M. (Also tried with --with-all-dependencies)"drupal/fullcalendar_view": "^5.1.12",composer require 'drupal/fullcalendar_view:^5.1.12' againClosed: cannot reproduce
5.1
Miscellaneous
Till Apr 2, 2023, there are 1,039 sites installed 5.1.12.
I am pretty sure some of them installed or updated to 5.1.12 via composer.
And I can't reproduce the issue reported.
So I close this issue for now. If there is a new finding, please feel free to re-open it.