SUMMARY
DVD cannot be played after link has been invoked from DVD's own menu.

STEPS TO REPRODUCE

$ dragon --play-dvd
*** Zero check failed in src/ifo_read.c:567
    for vmgi_mat->zero_6 = 0x0000000d00000000000000000000000000000000000000000000000000000000
libdvdnav: Using dvdnav version 6.0.1
*** Zero check failed in src/ifo_read.c:567
    for vmgi_mat->zero_6 = 0x0000000d00000000000000000000000000000000000000000000000000000000
libdvdnav: DVD Title: <Impys Island>
libdvdnav: DVD Serial Number: 3f026a04
libdvdnav: DVD Title (Alternative): 
libdvdnav: DVD disk reports itself with Region mask 0x00fd0000. Regions: 2

libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a _long_ time, please be patient

libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x00000190
libdvdread: Elapsed time 0
[...]
libdvdread: Found 16 VTS's
libdvdread: Elapsed time 0
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::begin: Paint device returned engine == 0, type: 1
QObject::connect: Cannot queue arguments of type 'QList<MediaController::NavigationMenu>'
(Make sure 'QList<MediaController::NavigationMenu>' is registered using qRegisterMetaType().)

** (dragon:13982): CRITICAL **: 14:51:45.177: gst_navigation_send_key_event: assertion 'g_strcmp0 (event, "key-press") == 0 || g_strcmp0 (event, "key-release") == 0' failed 

SOFTWARE/OS VERSIONS
Operating System: Fedora 31; KDE Plasma Version: 5.17.5; KDE Frameworks Version: 5.66.0; Qt Version: 5.13.2; Kernel Version: 5.4.15-200.fc31.x86_64; Component: dragon.x86_64 19.04.3-2

ADDITIONAL INFORMATION
Installed packages relevant for DVD playing:

$ dnf list installed ffmpeg gstreamer1-plugins-{bad-freeworld,base,good} libdvdcss lsdvd | sed '1d'
ffmpeg.x86_64                            4.2.2-1.fc31    @rpmfusion-free-updates
gstreamer1-plugins-bad-freeworld.x86_64  1.16.1-1.fc31   @rpmfusion-free        
gstreamer1-plugins-base.x86_64           1.16.1-1.fc31   @anaconda              
gstreamer1-plugins-good.x86_64           1.16.1-2.fc31   @anaconda              
libdvdcss.x86_64                         1.4.2-2.fc31    @System                
lsdvd.x86_64                             0.17-9.fc31     @fedora

$ regionset /dev/sr0
Current drive parameters for /dev/sr0:
  RPC Type: Phase II (Hardware)
  RPC Status: active region code (bitmask=0xFD)
  Drive plays discs from this region(s): 2


However on same computer, using Totem –by Gnome– v. 1:3.34.1 still in Plasma environment, DVD can be played.

$ totem
libdvdread: Attempting to use device /dev/sr0 mounted on /run/media/yk/<Impys Island> for CSS authentication
*** Zero check failed in src/ifo_read.c:567
    for vmgi_mat->zero_6 = 0x0000000d00000000000000000000000000000000000000000000000000000000
libdvdnav: Using dvdnav version 6.0.1
libdvdread: Attempting to use device /dev/sr0 mounted on /run/media/yk/<Impys Island> for CSS authentication
*** Zero check failed in src/ifo_read.c:567
    for vmgi_mat->zero_6 = 0x0000000d00000000000000000000000000000000000000000000000000000000
libdvdnav: Can't read name block. Probably not a DVD-ROM device.
libdvdnav: vm: dvd_read_name failed
libdvdnav: DVD disk reports itself with Region mask 0x00fd0000. Regions: 2

libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a _long_ time, please be patient

libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x00000190
libdvdread: Elapsed time 0
[...]
libdvdread: Found 16 VTS's
libdvdread: Elapsed time 0