The mirror modifer does not seem to be applied dynamically to animations in other formats, as shown in the gif (this is an animation where I’m moving in the timeline, not just rotations). Any suggestions for fixes or workarounds?
Welcome 
…
This is a bit puzzling…
…of course the modifer is a blender feature and to “export” the behaviour you have to apply it… some exporters even do this for you for the final result… so you can keep it… additionaly this seems a bit weird because you mirrored part (right) moves like the left… or so you have this already mirrored and use an additional mirror modifier ?? … ( don’t laugh… i have seen such a thing…).
Also ( as other newbies also do 
): If you just show the result and nothinh of your setup then any advice would be nothign else than just guessing…
Thanks for the advice 
The forum would only let me post one pic / gif since I’m a new user. Do you have any particular exporter you recommend?
Not any in particular… i just was thinking of the (new) OBJ exporter (which does this…actually it seem it’s the only one !?) but not the legacy OBJ exporter…
…but of course you just can apply it before export by yourself…