I followed the steps you provided to adjust the font size to 6px font in the field for mobile adjustments, but there was no change in the display on my phone, even though I cleared the cache repeatedly.
I also tried to adjust the font size display for H2, but encountered the same problem. Although I could see the changes when adjusting in the backend, there was no actual change on my phone. The only way I can make adjustments is through the editor in the article, which is quite troublesome.
You can check again