grep - 'Grotty' man page messages

If I try to search within certain man pages using grep, I get several 'grotty' messages.

For example:

man mpv | grep dvdnav

results in:

mpv dvdnav://[longest|menu|title][/device] [options]
grotty: ():90690: character above first line discarded
grotty: ():90694: character above first line discarded
grotty: ():90698: character above first line discarded
grotty: ():90702: character above first line discarded
grotty: ():91895: character above first line discarded
grotty: ():91899: character above first line discarded

grotty: ():91903: character above first line discarded
grotty: ():91907: character above first line discarded
grotty: ():94450: character above first line discarded
grotty: ():94454: character above first line discarded
grotty: ():94458: character above first line discarded
                                                  │dvdnav://              │ dvdnav://menu                         │

What causes these messages and how do I prevent them?

I have no mpv installed, but this seems more a warning of grotty (the program that process man page sources to print it to standard output) probably coming from no-compliant man pages. Have you tried

man mpv 2> /dev/null | grep dvdnav

? This will redirect error and warnings from grotty to the sink, so that you should have a cleaner output.

Nevertheless, that error is probably a bug in the mpv man page (or in grotty, you never know).