I should add that, with the minimal dictionary and .aff file,
"vertrags" gets reduced alright, dropping the trailing 's':
tstest=# SELECT tsearch2.ts_debug('vertrags');
ts_debug
---------------------------------------------------------------------
(german,lword,"Latin word",vertrags,"{ispell_de,simple}",'vertrag')
(1 Zeile)
The affix is just not applied while looking for compound words.
Sincerely yours
Alexander Presber