In an informal style, we sometimes mix singular and plural forms when we use demonstratives with kind, sort or type. I don’t like those kind of boots.

I got this from Practical English Usage, by Michael Swan, which I consider a great source. Its approach to language is descriptive, rather than prescriptive. That is real usage, but it can make things confusing if you are aiming for a formal language certification. So, it is above proficiency, to put it that way. Of course, Grammarly doesn't agree with Swan. I would say that, in case of doubt, the best is to stick to what is logical grammatically. But anyway, it's worth checking the book mentioned. There are many common exceptions in real usage when it comes to the use of singular or plural forms. To get an idea of how often a "wrong" expression is used, you can always check the phrases in Youglish and similar websites.

In an informal style, we sometimes mix singular and plural forms when we use demonstratives with kind, sort or type.

I don’t like those kind of boots.

I got this from Practical English Usage, by Michael Swan, which I consider a great source. Its approach to language is descriptive, rather than prescriptive. That is real usage, but it can make things confusing if you are aiming for a formal language certification. So, it is above proficiency, to put it that way. Of course, Grammarly doesn't agree with Swan. I would say that, in case of doubt, the best is to stick to what is logical grammatically. But anyway, it's worth checking the book mentioned. There are many common exceptions in real usage when it comes to the use of singular or plural forms. To get an idea of how often a "wrong" expression is used, you can always check the phrases in Youglish and similar websites.