Why does the house temperature on my AC thermostat read out at 87 degrees long after sundown?

multiple causes:  1) your a/c isn't working properly and needs a technician.  2) your ceiling [attic] insulation is far too little

Sounds like your house is well insulated.  It gets heated up by sunlight pouring in through the windows during the day and retains the heat generated by that sunlight after the sun sets.

I have a similar situation.  But my house gets to 77 F and then stays there because I also have air conditioning to prevent the temperature from going higher.  But after the sun sets, the house stays at 77 F for several hours before it starts to cool down from there.  I have very good insulation in the attic.