It looks very much like what happened to me in 12c :
MaidSafe Dev Update - March 2, 2017 - Test 12c - #79 by ustulation
A new config option was added to the crust config file , which should adress the case of some weird router behaviour :
Fix timer bug by ustulation · Pull Request #825 · maidsafe-archive/crust · GitHub
Hopefully setting “force_acceptor_port_in_ext_ep” to TRUE should fix this.
I did not have a chance to try it yet as I am away from home until this afternoon.
EDIT
@ustulation , force_acceptor_port_in_ext_ep works perfectly my side . If I set it to false, test 15 behaves just like 12c and is not reachable from outside. Setting it to TRUE makes it visible. GOOD JOB ! 