I have had this problem with roxygen2 as well. Couldn't see any problem with any of my functions. In the end deleting the .rdb file and then getting roxygen2 to rebuild it seemed to solve the problem.
Basically all answers require restarting R to resolve the issue, but I found myself in an environment where I really didn't want to restart R.
I am posting here a somewhat hack-ish solution suggested by Jim Hester in a bug report about the lazy-load corruption issue.
The gist of it is that the package may have some vestigial S3 methods listed in session's .__S3MethodsTable__. environment. I don't have a very systematic way of identifying which S3 methods in that environment come from where, but I think a good place to start is the print methods, and looking for S3method registrations in the package's NAMESPACE.
You can then remove those S3 methods from the .__S3MethodsTable__. environment and try again, e.g.