I imported a new project from my bitbucket.org account and It worked properly. After several days working, doing commits, push, pull, now I can not see the Git tab.
If I create a new project it works well.
Very interesting. Are you able to share the URL for the project?
I am not 100% sure how this works on RStudio Cloud, but I know that in the IDE you need to be in a git repo to show the git tab. You probably have the project open, so this shouldn't really be an issue, but what does pwd or ls -lha or git status give you in the terminal? It's important to be sure that nothing happened to your git repo.
You can access the shell by using the Terminal tab in RStudio. See the screenshot below.
However, I suspect the issue here is that you need to open the project file to re-enable Git support. You can do this by navigating to project (.Rproj) file in the "Files pane, and opening the project file for your GitHub project.