Why a module has a package for a 8.x-3.0-beta1+54-dev version?
The 8.x-3.0-beta1+54-dev version means the development snapshot is 54 commits ahead of the 8.x-3.0-beta1 version. Similarly, 8.x-1.1+1-dev means the development snapshot is one commit ahead of the 8.x-1.1 version.
Since development snapshots can change much during time, it is probably a way to make better understand how much the development snapshot changed.
Notice that 8.x-3.0-beta1+54-dev is the version reported from the page listing all the installed modules or in the .info file for the module (as you shown, between the lines added by the Drupal.org packaging script).


(The screenshot is for Drupal 7.)

It's not the title you see for a release node in the page showing all the releases (for the Field Group module, in this case), where the release is field_group 8.x-3.x-dev.
To answer your question, it's not a custom syntax. It's the version that the Drupal packaging script adds in the .info file for development snapshot releases, which could then be used in the user interface when showing the installed project version.
The Drupal.org packaging scripts alter the .info file when creating the archive for the project. If the project is copied using Git or Composer, for example, that information is not present, and the version wouldn't appear as 8.x-3.0-beta1+54-dev. (This is also the reason why I took the screenshot from a Drupal 7 instance created on simplytest.me.)