ffplay won't play internal(embedded) subtitle?
You can use subtitles filter like your example in your question but point to the same input file:

ffplay -vf subtitles=input.mkv input.mkv


If you want to choose a different subtitle stream then use the stream_index option. This example will choose the third subtitle stream:

ffplay -vf subtitles=input.mkv:stream_index=2 input.mkv


Or use mpv player and press j or use --sub=1 option.