Grammarly tool does not support SharePoint and Office 365 page editor. It will save breaking HTML codes and scripts to the page content. Usually, it does not save anything, but because it does not support SharePoint, the codes will not be removed and will cause your SharePoint pages to behave in a very weird way.
For example, you will notice random numbers printed out in your pages.
Some webparts will stop working.
Pages will show browser crash messages
Disable Grammarly on all your SharePoint sites