By default, the rich text editor is turned off in the Bulk Product Edit app. Turning it on allows you to paste or type plain text (non-html text) into the editor without worrying about adding html code to format it, as the rich text editor will automatically format your text by adding HTML tags to your plain text.
However, if you are going to paste or type html text into the editor, you should turn the editor off by clicking on the icon shown in the image below. If you don't turn editor off, it will add additional html code on top of your html text causing conflicting html formatting to your text.
When it's turned off, you will no longer see the formatting options at the top, such as bold, italics and underline. It looks like this screenshot below:
Once it has been turned off, you can either paste or type your html text. Please do not toggle the html editor back on. If you do that, more html code will be added to your text.
In summary, if you input html text, turn off the rich text editor, if you input plain text, turn on the rich text editor.