First check to make sure that the arm was set up properly.  See here for Benchmark installation instructions.

It has to do with how the control senses the motor current when pushing the door into the hold open position at 90°.  To fix in the field - try to increase Backcheck Speed and also decrease Backcheck position until you get the hold open to function as desired.

Also, if you have an Electric Strike, the ES Filter Diodes, which are included with the operator, must be installed over the leads of the electric strike to prevent back-feed into the controller which will also temporarily erase hold open time.  For ES wiring instructions see here.