I just tried to send audio to a plugins side chain and for some reason it didn't work. After I moved it to another track it worked, so I tried putting another plugin with a side chain on a bus and I got the same results. It seems the side chain function doesn't work when the plugin is on a bus. Can anyone else confirm this? Is this a bug?

I found a few earlier references to this issue on the forum, but no solid confirmation that it's a known issue or has been reported. But I can repro it in both Platinum Everett and X3e so it's not new issue. Page 1043 of the Platinum Ref. Guide indicates it should work:
 
"Insert the sidechain-capable plug-in in a track or bus FX Rack..."
 
 

Yeah, sidechain has issues.

Correction. My test case was no good. I found it was due to not having any signal going to the bus. I initially tested with a plugin that just passes input from the side-chain into the bus via the FX bin, and that's not enough to "energize" the signal path. When I side-chained a compressor and actually gave the bus an input signal to work on, the bus woke up and started passing both signals.
 
I think it's considered to be working as intended that buses aren't active unless they're getting a signal at the Input.
 
Possibly this explains the OP's issue as well.