Atlas texture not appearing on MeshInstance3D until project saved & restarted

[ChurchGargoyle]

Tested versions

Reproducible in: v26.2.stable.official [4f5b14a], v26.1.stable.official [a3e8531]

System information

Redot v26.2.stable - Windows 11 (build 26200) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 5060 Laptop GPU (NVIDIA; 32.0.15.9186) - AMD Ryzen 9 8940HX with Radeon Graphics (32 threads) - 15.21 GiB memory

Issue description

Atlas texture not appearing on MeshInstance3D when applied until the engine is saved and restarted.

Steps to reproduce

Create MeshInstance3D
Make mesh type (I tried QuadMesh and BoxMesh only)
Add material StandardMaterial3D
Then Albedo -> Texture -> AtlasTexture
In AtlasTexture add Atlas -> Load
Take the texture (like icon.svg, also tested .png)
It applies to Atlas but does not appear on MeshInstance3D
Save project and quit, then restart again
Texture now appears on MeshInstance3D

Issue_AtlasTex_Demo.mp4

Minimal reproduction project (MRP)

atlas_issue.zip

[JoltedJon]

Confirmed on Dev build of 26.3 44ce9f4