The summary is that when you set up new content packs in your " build.gradle " and create all the folders, any vehicles in your custom packs render backwards when held in the players hand and are invisible when placed ( with exception to their shadows. ) I have checked my grammar multiple times in the build.gradle, how i reference the locations in the model files and configurations, etc and I have not found any discrepancies in all of my attempts. This only happens with vehicles such as ones that fall under the config option of Plane, Heli, Tank etc, not mechas or any guns. There is a short clip below showing the issue in game.

Steps to reproduce the behavior:

1. Add the necessary lines to your build.gradle for a FM 1.12.2 workspace.
2. Create all the required folders and reference the new paths in the model and configuration files for the new content pack.
3. Compile the workspace and add the content pack to an MC instance.
4. Run the instance, load up a world and drag a vehicle from the pack you recently made into your hot bar. It will be held and placed oddly.

Demonstration of the error:
https://youtu.be/he_-SkeH37U

Forge: Forge 1.12.2-14.23.5.2847
Flan's Mod version: Flan.s.Mod-1.12.2-5.6.0-beta.7.jar

NOTE: This only seems to happen with content packs that are added brand new (so if there are 5 packs that come in the default build.gradle, the sixth pack and the packs to follow are the only ones to do this. as well land vehicles seem to cause a world crash rather than just being invisible, but this may be an issue on my part with migrating my land craft content.