I've fixed the rubberbanding issue (the tendency to be teleported in a random direction when dismounting a speederbike) for myself with the following macro.
For some reason, performing an emote macro seems to synchronise your avatar's position on the client and server so you don't bounce off when you dismount.
I use the /applaud macro but you can use others like /wave etc. You can also add in lines like /target Speederbike and /mount so that you only have one macro for mounting and dismounting.
/applaud;
/pause 2;
/dismount;
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