If you see a 'flicker' when you open your website in a browser (which means that at first, the system font is loaded, then your custom font), it's not an error. This is perfectly normal.
It takes time for a file to load. Sometimes it happens instantly (especially if you have fast internet), and you see your custom font straight away. Sometimes there is a short pause.
For a fast page load time so your visitors get what they want as quickly as possible, content – the most important element – loads first, followed by all the additional elements. This is why you may see the system font before your custom font is loaded.
You may add the 'Page load effect' block to your page (Т228, Other category). It will slow down page load speed, and the browser will display text with the custom fonts.