Rankmath auto add %sitename%?

Hello @huyhoanet

Thank you for contacting the support and sorry for not following up quickly and any inconvenience that might have been caused due to that.

That’s completely normal for Google to include your brand name in the titles automatically – even if you deliberately leave them out from your titles.

Here’s an article mentioning that as well.

https://seo-gold.com/marcus-lemonis-website-seo-review/google-automatically-adds-brand-name-to-titles/

And a link to Google’s forums with replies from Experts saying that it is like that on purpose:
https://support.google.com/webmasters/thread/5328043?hl=en&msgid=5328043

If you wish to know Google’s guidelines on writing Good titles, here are the guidelines straight from the horse’s mouth:
https://support.google.com/webmasters/answer/35624?hl=en

With that said, it largely depends on keyword intent too. For some keyword searches, your custom title might appear while for others – Google might append your brand name to the title and there is nothing you can do to force them to do otherwise. That is regardless of what SEO plugin you use.

Hope that helps and please do not hesitate to let us know if you need our assistance with anything else.