HELP! Something about the "mark" tag went wrong!

Unity 2022.3.4f1
Open a new project -> Create UI->Text - TextMeshPro -> Import TMP Essentials
When I simply enter [quote]
This text is highlighted Over
[/quote] , it looks like this
9489298--1335472--upload_2023-11-23_21-53-18.png
However,when I change the LiberationSans SDF.asset like this 9489298--1335475--upload_2023-11-23_21-53-50.png

it turns out to be like this
9489298--1335478--upload_2023-11-23_21-54-28.png

How weird! The "mark" tag doesn't make sence at all! Anyone can explain this pls?

also,when I cahnge the LiberationSans SDF.asset like this
9489298--1335481--upload_2023-11-23_21-56-59.png
it turns out to be like this
9489298--1335484--upload_2023-11-23_21-57-12.png

I cannot get rid of this! It looks like so strange and makes me MAD!

For me it is entirely covering the text like your first example, which I thought was how it worked if you dont have alpha in the color but was therefore useless to me. If it could actually work like the bottom image, so that its behind the text, id find that quite useful.