CSS not loading for tag since a while ago

Previously I created a CSS class for the <mark> tags on my posts.

.marker-pen {
    background: linear-gradient(to top,PaleTurquoise 30%,transparent 0%);
    font-weight: 700;
}

The HTML part is <mark class="marker-pen">content</mark>
The <mark> tag can be styled without any problems, however this week I found out that they all turned into the default style (yellow background). class="marker-pen" remains in the html source code, but I couldn’t see .marker-pen anymore in the DevTool.

The issue is relardless of the themes I use. I really have no idea what has changed and how can I fix it?
The page I attached contain a few lines with customized <mark> (now having default style with yellow background)

The page I need help with: [log in to see the link]