$paged value being set to 0

I’m running some custom URL filters to allow myself to create some nice looking URLs.

Specifically:

‘(sites)/(.+?)/(page)/?([0-9]{1,})/?$’] = ‘index.php?pagename=$matches[1]&site_id=$matches[2]&paged=$matches[3]

[‘(sites)/(.+?)$’] = ‘index.php?pagename=$matches[1]&site_id=$matches[2]

The URL works correctly but the paging is not. The $paged value in the $wp_query object is always 0.

So this URL shows the first page of results

https://localhost/blog/sites/14/

but this doesn’t

https://localhost/blog/sites/14/page/2

Oddly this will work

https://localhost/blog/page/2/?pagename=sites&site_id=14

as will

https://localhost/blog/index.php?pagename=sites&site_id=14&page=2

Any pointers to what might be causing the issue?

TIA