Finally! I've managed to reproduce the failure. Only occurs about 10% of the time, at random.

The cause is a race condition while loading layers. List<> is not thread-safe, but is being called without first being locked. As a result, Layers can be lost when the List resizes itself. This particular file has 225 layers with dimensions of 24 x 24 each. Because the layers are so small, the LoadLayersContext tasks are completing so quickly that they have a chance of racing with the outer for loop.

Thanks for reporting the bug! Nondeterministic bugs are the hardest sort to track down.