The first time NotifyPreAppend is called on an array it will usually fail this check:

void NotifyPreAppend()
{
    check(PreviousCount == INDEX_NONE);
    PreviousCount = NativeSize();
}

PreviousCount is not initialized in this class, and so in optimized builds its initial value is whatever was sitting in that spot in memory.

Unreal 5.4.4, Noesis 3.2.8, DebugGame build configuration.