Circa unfortunately it's a complex issue deep into the Flarum javascript translation templates that causes this flarum/framework3685
The only "solution" would be to modify the page title template for your language in such a way that the title has the same number of tokens no matter the plural rule. The easiest way to achieve this is to entirely remove the plural rule and therefore the part that mentions the page number. The page title translation template can be modified with the Linguist extension or by registering a custom translation file.
The title plural rule for page numbers are only visible to search engines I believe, but the fact it's used to generate the page titles leads to this issue.
Maybe it's also worth noting that most search engines will see the correct ampersand, because the issue only happens once the javascript single page app boots. The page HTML itself should contain the expected value.
