Good point.
In my case, DC has a no-chase policy for bikes & ATVs (riders were going on the sidewalks endangering ppl to avoid police), I also ride in Va & MD and got pulled over once but when I showed him that I did indeed have a plate and where the plate was he let me go no ticket.
What part of PA you in?
From: DMV.DC.GOV
Vehicle Tags
Vehicles registered in the District of Columbia are required by law to display DC DMV-issued vehicle tags. Tags will be issued at registration. Vehicles are required to display two current tags, one on the front and the other on the rear of the vehicle.
Exceptions are for motorcycles, mopeds, and trailers. For these, one tag is issued to be displayed on the rear of the vehicle.
DC DMV tags must be attached to the vehicle to which they are issued. They must be securely fastened in a
horizontal position, so that they do not fall off and can be easily read. District law requires that the tags be visible and not obstructed or covered by any substance (e.g., glass, plastic, spray, license tag holders, etc.).
Sounds like you got lucky getting the cop that pulled you over? Others maybe not so much.....