I just tried both for me. Both do not work.
For a planned route, the fly-through does nothing. You are most likely writing some values with the current system locale.
For some countries, this means that there is a decimal comma instead of point. However, KML expects a point. This affects the gx:duration tag in KML (24,19 instead of 24.19). Replacing these commas with points makes the fly-through work:
<gx:FlyTo>
<gx:duration>24,19</gx:duration>
<gx:flyToMode>smooth</gx:flyToMode>
<Camera>
<longitude>-2.0356</longitude>
<latitude>46.6632</latitude>
<altitude>1012</altitude>
<heading>130</heading>
<tilt>80</tilt>
<altitudeMode>absolute</altitudeMode>
</Camera>
</gx:FlyTo>
<gx:FlyTo>
<gx:duration>24,19</gx:duration>
<gx:flyToMode>smooth</gx:flyToMode>
<Camera>
<longitude>-2.0173</longitude>
<latitude>46.6527</latitude>
<altitude>1012</altitude>
<heading>130</heading>
<tilt>80</tilt>
<altitudeMode>absolute</altitudeMode>
</Camera>
</gx:FlyTo>
When trying to do a fly-through for a past flight, a kmz is generated. When viewing that,
there is only one gx:duration tag overall - in the first gx:FlyTo section. Later FlyTo sections do not contain a gx:duration tag at all. Therefore the one second route that someone else mentioned earlier.
<gx:Tour>
<name>Track Log Fly-Through</name>
<gx:Playlist>
<gx:FlyTo>
<gx:duration>1</gx:duration>
<gx:flyToMode>smooth</gx:flyToMode>
<Camera>
<longitude>10.2794</longitude>
<latitude>48.4853</latitude>
<altitude>441</altitude>
<heading>180</heading>
<tilt>80</tilt>
<roll>0</roll>
<altitudeMode>absolute</altitudeMode>
</Camera>
</gx:FlyTo>
<gx:FlyTo>
<gx:flyToMode>smooth</gx:flyToMode>
<Camera>
<longitude>10.2816</longitude>
<latitude>48.486</latitude>
<altitude>447</altitude>
<heading>60</heading>
<tilt>80</tilt>
<roll>0</roll>
<altitudeMode>absolute</altitudeMode>
</Camera>
</gx:FlyTo>
Thank you for fixing this.