Ok, problem was about 60cm in front of the PC.
My notebook was set to Chinese local, thats why the POI MRT didn't show up in the "Where to" screen, also thats why the POI's showed only distance and nothing else.
After set the notebook to English local, the MRT symbol showed up also the description.
mrtc
Nevertheless on the map still no symbols
nomapsymbols
But now the most weird part
Taiwan maps have an additional House Number Search, which is easier to find addresses
housenumbersearch
As soon as I copy the gpi File to the POI folder the house number search is gone
nohousenumbersearch
I can duplicate this, delete the gpi file, house number search is available, copy the gpi file to the POI folder, the house number search is gone