When the dark theme is enabled on iOS and the link is opened in Telegram, the link opens in the platform's built-in browser. And if the background on the website is white and the text is black, then the text is not visible, since the dark theme makes the website black. (the problem is displayed on the dark screenshot, there should actually be text there).
At the same time, if you open the site in a standard browser that is not embedded in Telegram, then everything is OK (this is screenshot 2, white. This is how the page should look like).
for some reason I couldn't upload screenshots, so I'm attaching links: