There is a scenario where delta struct serialized fast arrays can replicate changes that introduce additional elements to the array, but without changelists containing the new data.
This needs further investigation to determine an isolated repro case. It is a regression against the original replication behavior of fast array serialization.
See linked UDN thread for more information:
https://udn.unrealengine.com/s/question/0D54z00006yDw2gCAC/fast-array-struct-delta-serialization-issues
See code below.
Expected:
In the log, you should see this, indicating that the fast array was replicated twice. The first time with 5 elements, and again with 10 elements. All elements should have valid Objects and Values.
[2022.01.15-19.12.28:834][779]LogTemp: Warning: AFastArrayTestActor::PostNetReceive = [{ Value = 1, Object = FastArrayTestActor_1 },
{ Value = 2, Object = FastArrayTestActor_1 },
{ Value = 3, Object = FastArrayTestActor_1 },
{ Value = 4, Object = FastArrayTestActor_1 },
{ Value = 5, Object = FastArrayTestActor_1 }]
[2022.01.15-19.12.28:835][779]LogTemp: Warning: AFastArrayTestActor::PostNetReceive = [{ Value = 1, Object = FastArrayTestActor_1 },
{ Value = 2, Object = FastArrayTestActor_1 },
{ Value = 3, Object = FastArrayTestActor_1 },
{ Value = 4, Object = FastArrayTestActor_1 },
{ Value = 5, Object = FastArrayTestActor_1 },
{ Value = 6, Object = FastArrayTestActor_1 },
{ Value = 7, Object = FastArrayTestActor_1 },
{ Value = 8, Object = FastArrayTestActor_1 },
{ Value = 9, Object = FastArrayTestActor_1 },
{ Value = 10, Object = FastArrayTestActor_1 }]
Actual:
In the log, you should see this, indicating that the fast array was replicated twice. The first time with 5 elements, and again with 10 elements. In this case, the first 5 elements have valid Values and Objects, but in the second case all Values are 0 and all Objects are None, meaning the structs are using default values.
[2022.01.15-02.42.15:295][108]LogTemp: Warning: AFastArrayTestActor::PostNetReceive = [{ Value = 1, Object = FastArrayTestActor_1 },
{ Value = 2, Object = FastArrayTestActor_1 },
{ Value = 3, Object = FastArrayTestActor_1 },
{ Value = 4, Object = FastArrayTestActor_1 },
{ Value = 5, Object = FastArrayTestActor_1 }]
[2022.01.15-02.42.15:295][108]LogTemp: Warning: AFastArrayTestActor::PostNetReceive = [{ Value = 1, Object = FastArrayTestActor_1 },
{ Value = 2, Object = FastArrayTestActor_1 },
{ Value = 3, Object = FastArrayTestActor_1 },
{ Value = 4, Object = FastArrayTestActor_1 },
{ Value = 5, Object = FastArrayTestActor_1 },
{ Value = 0, Object = None },
{ Value = 0, Object = None },
{ Value = 0, Object = None },
{ Value = 0, Object = None },
{ Value = 0, Object = None }]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-118168 in the post.