I believe there was a Forum post about this before, however I still encounter this. The forum stated the bug was fixed in 1.9 client, which seems to be true. However, my server still supports 1.8 due to the vast amount of the community that still uses 1.8 rather than 1.9 :\

Forum link here: https://www.spigotmc.org/threads/arrows-flopping.23904/

Note: in the post link above, it shows the arrow stopping mid air... Which is not exactly what happens on my server, it seems to fly a random direction but actually hits the correct location.

I've NEVER seen this bug on any other 1.8 server except for my own.

Thanks,
Swedz