AssetGenerator: save simple assets (MaterialInstance, MPC, PhysMat, DataAsset...) after data population - 5.6.1 regression

[UresiiZo]

Symptom

Since the UE 5.6.1 update (d665fd6) every asset handled by USimpleAssetGenerator and its subclasses
(UMaterialInstanceGenerator, UMaterialParameterCollectionGenerator, UPhysicalMaterialGenerator,
UDataAssetGenerator, UCurveBaseGenerator, ...) is written to disk without its data:

• MI_Foundation_Concrete.uasset = 1,847 bytes, Parent = None, TextureParameterValues = []
• MPC_Panini.uasset = 1,041 bytes (no parameters), PM_FreeLinearAngular.uasset = 1,119 bytes
• Full Satisfactory dump (UE 5.6.1-CSS, 18,544 packages): 2,034 of 2,042 MaterialInstanceConstant packages
  were empty shells (file <= 2,000 bytes; the 8 others got dirtied by another generator), 10 of 12
  MaterialParameterCollection, all 25 PhysicalMaterial, all 674 FGMessage, 255 FGUserSetting,
  221 AnimMontage, 98 CurveFloat, ...

Meshes / textures / materials / blueprints are fine, so every vanilla mesh shows up grey in the editor.
No error is logged; every re-run prints MaterialInstanceConstant ... is not up to date, regenerating data
for the same assets, but the file on disk never changes (its mtime stays at the CONSTRUCTION-stage save).

Cause

d665fd6 moved the population of simple assets out of USimpleAssetGenerator::CreateAssetPackage()
(CONSTRUCTION stage, which always calls MarkAssetChanged() in UAssetTypeGenerator::ConstructAssetAndPackage)
into the new USimpleAssetGenerator::PopulateAssetWithData() (DATA_POPULATION stage). That method never calls
MarkAssetChanged(), and neither UObjectHierarchySerializer nor UPropertySerializer dirty the package.
UAssetTypeGenerator::AdvanceGenerationState() only saves when bAssetChanged is set, so:

1. CONSTRUCTION creates the empty object and saves it (this is what ends up on disk),
2. DATA_POPULATION deserializes Parent / parameters into memory,
3. nothing saves it; the generator finishes and GC drops the populated object.

UMaterialInstanceGenerator::PreFinishAssetGeneration() has the same problem: it deserializes AssetUserData
in PRE_FINSHED, after the DATA_POPULATION save.

Fix

• SimpleAssetGenerator.cpp: call MarkAssetChanged() right after PopulateSimpleAssetWithData() in PopulateAssetWithData().
• MaterialInstanceGenerator.cpp: call MarkAssetChanged() at the end of PreFinishAssetGeneration().

Verification (real dump, UE 5.6.1-CSS, Windows, -run=AssetGenerator commandlet)

Targeted re-run of 3 floor MIs first (-ForceGeneratePackageNames, everything else blacklisted; 1.6 s):
MI_Foundation_Concrete.uasset 1,847 -> 7,702 B; editor Python read-back: Parent = MMI_FoundationSet_Concrete,
Albedo = TX_Concrete_BC, Normal = TX_Concrete_N, Masks = TX_Concrete_Relf, MaskAO = TX_Concrete_AOMasks;
MMI_FoundationSet_Concrete.Parent = MM_BakedStencil_01.

Then a full re-run over the whole dump with the fix (commandlet time 4 min 48 s, 18,347 packages generated,
no crash). Empty-shell counts (file <= 2,000 B) before -> after:

class	total	shells before	shells after
MaterialInstanceConstant	2,042	2,034	0
FGMessage (DataAsset)	674	674	0
FGUserSetting (DataAsset)	255	255	0
AnimMontage	221	221	0
MaterialParameterCollection	12	10	4 (*)
CurveFloat	98	98	98 (*)
PhysicalMaterial	25	25	25 (*)

(*) these are just small assets: all of them were rewritten by the fixed run with a larger size, and the
editor reads their data back (e.g. PhysMat_Cement.SurfaceType = SurfaceType11, DebugColor set; MPC with
zero parameters: 0 / 12).

Cross-check from the log: 3,671 packages logged is not up to date, regenerating data; 3,661 of them now
have a rewritten file on disk. The 10 that do not are all CurveLinearColorAtlas, which uses its own
generator (UCurveLinearColorAtlasGenerator, populated in CONSTRUCTION, so it was never a shell) and has the
same missing MarkAssetChanged() in OnExistingPackageLoaded() - left out of this PR to keep it minimal.

Editor Python read-back over all 2,042 MaterialInstanceConstant assets: Parent == None = 0. One MI
(MI_Liana_Ivy_01) has the engine default parent - that is the existing "Failed to deserialize Parent Material
... Falling back to default material" path, unrelated to this bug. A random sample of 8 MIs shows parent,
texture, scalar and vector parameters matching the dump.

Steps to reproduce (before the fix)

1. Dump the game with AssetDumper, run -run=AssetGenerator -DumpDirectory=... on a UE 5.6.1 project.
2. In the editor: unreal.load_asset('/Game/FactoryGame/Buildable/Building/Foundation/ConcreteSet/MI_Foundation_Concrete').get_editor_property('parent')
   -> None; file size ~1.8 KB.

Possibly related

Reports on the modding Discord (2026-05) of "followed the docs, everything in the editor is still white cubes /
grey" after generating with the 5.6 toolkit may have the same root cause (material instances empty ->
default material), though I have not verified those cases.