This name is a synonym of Rosa marginata Wallr..
The record derives from RJP
(data supplied on 2011-10-18)
which reports it as a synonym (record 48826) with original publication details: Conspectus florae europaeae Conspectus florae europaeae; 1878 234 1878.
This name record (from RJP) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Rosa flexuosa in version 1 of The Plant List.