Some signals became ? and missing on the simvision, why?

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I am performing a huge simulation, netlist and SDF file size were each
~110MB. The first segment of my simulation were correct, then all of a
sudden some of my signals became question marks. On the simvision,
they appear to have been blank after certain time, and cursor value turned
question mark.

Is this caused by memory depletion or what?

Thank you for your input.