The state of the Ruler toggle is a setting that is store in the Work Environment.
If you close ArchiCAD then restart it and you use the last Work Environment you used, the Ruler should stay displayed.
If you switch to another Work Environment in the Start ArchiCAD Dialog, the Ruler could get turned off.
If this is not how it works for you it could be a bug.
However, I tried it on AC15 just now and it works correctly for me.
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