I did not reproduce it. It probably depends on what is in the child project. You can attach a log here, but it probably will not help. With that said, what are trying to do it incredibly inefficient usage of the engine because every split adds another complete copy of the project. Every time you open the parent project, it must open everything inside the child project N times. If you need to split a child project or include it more than once, you should consider to render it to an intermediate and use that.
Thank you for your answer. I see, it demands a lot of math for my machine
Actually I did’nt want to render to avoid quality loss. I thought that it would be like a simple reading of the .mlt file