Applying for pure math PhD programs without a math degree

[kkkkat]

I'm about to graduate in the summer of 2020 with a STEM degree in a reputed university in asia.
I consider myself a math major in a sense, without a degree however. I took about 20 math courses, 8 of which are graduate courses.
I also have a year-long research experience in pure math at my home institution, without anything publishable though.

Do you think my major not being math a problem? Should I try to make up for it in my PS or SOP?
Do I just mention it with a couple of sentences, or use a paragraph (or more) to talk about my move from my major to math?

Any suggestions are welcome. Thanks in advance!

By the way, I have got a 98% in GRE subject math, if that helps.

[temporaryacct]

[deleted]

[kkkkat]

Thank you for the information.

If you have the course requirements that schools list, then the name of your major doesn't matter too much.