Plugin Registration Tool not showing the new plugins

Hi,

CRM 2016 On premise.

We have a set of plugins in a project. We tried to add a new plugin class and update the plugin through registration tool. When I select the assembly and click load assembly it does not show the newly added plugin (Class). The class and execute method is declared as public. But I don't know why the new class is still not showing up. To investigate further I remove an existing plugin (Class). Then when I load the assembly to deleted  plugin is still showing. How does the registration tool shows a class which does not exist in the assembly. It looks the plugin registration tool is loading from cache. I restarted the IIS, server without success. This look strange. Please help.