Make sure that the extproc.exe is not running in database server when you redeploy your assembly. You can stop it using OraClrAgent service. You may also manually check if your assembly in database server at <OracleHome>\bin\clr is updated after redeployment.
I was able to resolve the problem by changing the assembly version and file version in the project properties. I'm not sure which one made it work, but it seems that Oracle wouldn't load in the new code if the version was the same.