Custom Class does not work

I tried to apply a custom class to a paragraph block. But every time I save my page, this custom class gets lost. Instead, the class ?ek-link? is added – not to the paragraph, but to the link that is contained in that paragraph. And this ?ek-link? is added not only once, but as many times as I tried to save my page. The only solution I found so far was to switch the block to HTML mode and add the class manually. (I am using Gutenberg 8.6.1)