Trailer seems to bounce when applying service brakes

Could be that the brake shoes and drums
Are not worn in (bedded in) and also new friction material is grabby and brakes are probably adjusted up tighter than tractor brakes. That is usually the case. Give them a few days snd it will settle down

 

If you haven’t replaced shocks recently like in the last year it would be a good idea.

 

Couple loaded panic stops should take care of it,,,

 

I have trailers transferred in here with those type problems. They often have the wrong drum and or brake shoes. Did not replace cam bushings as needed.
Hope a little running and proper adjustment works for you.
Call the people that did the work and ask them their opinion. Then there is a record of it.