This is not a bug, but rather a result of how the level of detail (LOD) transition system for models is deisgned for most, if not all, games. LODs are an optimization technique in video games that reduces the complexity of 3D models and textures as they get further from the player. In Arma Reforger, the distance between when an LOD transitions from one LOD to the next LOD (eg, LOD0 -> LOD1, LOD1 -> LOD2, ..., LODX -> Derendered) depends on a model's volume. As you can see in the attached image, the BTR-70 and the M923A1 trucks have a greater volume than LAV-25 and Ural-4320 trucks, respectively.

Point is, its not a bug or accident. The system is designed to do this for all models as a way of optimally managing vertex/triangle count to improve the game's FPS. Bigger models require greater distance between the player's camera and the model to derender compared to smaller models.