Looks like the newly-built mp4tools didn't pick up the correct version of the also-newly-built wxsvg, as it contains an incompatible symbol definition.

With both installed:

mp4tools-3.7-3.fc29.x86_64

wxsvg-1.5.16-1.fc29.x86_64

$ mp4joiner

mp4joiner: symbol lookup error: mp4joiner: undefined symbol: _ZN20wxFfmpegMediaDecoder11SetPositionEdb

$ nm -D /lib64/libwxsvg.so.3|grep -i FfMpegMediaDecoder11SetPosition

000000000011fa30 T _ZN20wxFfmpegMediaDecoder11SetPositionEdbb

Note how the symbol ends with "Edb" (mp4tools) vs. "Edbb" (wxsvg).

$ sudo dnf downgrade wxsvg

Downgraded:

  dvdstyler-1:3.0.4-5.fc29.x86_64           wxsvg-1.5.12-7.fc29.x86_64          

$ mp4joiner

[dumps core, complaining about...]

(mp4joiner:1172): Gtk-ERROR **: 16:18:56.917: GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported

zsh: trace trap (core dumped)  mp4joiner

$ sudo dnf downgrade mp4tools

Downgraded:

  mp4tools-3.6.1-2.fc29.x86_64                                                  

$ mp4joiner  [...works]