SubViewport only draws once, only in Safari Technology Preview

[mahalis]

Tested versions

• Reproducible in: 4.3 stable + Safari Tech Preview 199
• Not reproducible in: 4.3 stable + regular Safari or Chrome

System information

Godot v4.3.stable - macOS 14.5.0 - GLES3 (Compatibility) - Apple M1 Max - Apple M1 Max (10 Threads)

Issue description

It seems that a SubViewport will only draw successfully once—after that it keeps its contents indefinitely, apparently ignoring the render_target_update_mode.

Steps to reproduce

1. Get the 2D in 3D demo project
2. Export it for web
3. Open the resulting page in Safari Tech Preview

Minimal reproduction project (MRP)

https://godotengine.org/asset-library/asset/2803