@wmr: Are all other Wikisource sites also affected by this?

In general, the page numbers shown in the gutter of transcluded texts (i.e. in mainspace / ns:0) are provided by a local (community-developed and -supported) Gadget. I took a quick look at zhWS's setup but couldn't immediately identify where that functionality would come from.

But provided my assumption above is correct this is something you'll need to find a technically-minded (JavaScript) community member to sort out, and is not related directly to MediaWiki or the Proofread Page extension as such.

Ah. I found it. zhWS used to import an old old version of the pagenumbers script from enWS, but it was removed last November by @WikiBayer apparently in response to a steward request. Probably because that script on enWS is very very old and has not worked for a very long time. enWS has migrated to an actual Gadget (MediaWiki:Gadget-PageNumbers.js, MediaWiki:Gadget-PageNumbers-core.js, and MediaWiki:Gadget-PageNumbers-core.css).

In T337707#8887945, @Aklapper wrote:

@wmr: Are all other Wikisource sites also affected by this?

no

In T337707#8888579, @Xover wrote:

Ah. I found it. zhWS used to import an old old version of the pagenumbers script from enWS, but it was removed last November by @WikiBayer apparently in response to a steward request. Probably because that script on enWS is very very old and has not worked for a very long time. enWS has migrated to an actual Gadget (MediaWiki:Gadget-PageNumbers.js, MediaWiki:Gadget-PageNumbers-core.js, and MediaWiki:Gadget-PageNumbers-core.css).

Now we know the cause. Can anyone fix it?

In T337707#8890799, @wmr wrote:

Now we know the cause. Can anyone fix it?

You'll need to talk to the Interface Administrators on zhWS, which is currently @hat600 and @Shizhao (Hat600/Shizhao: feel free to hit me up on-wiki if I can assist in adapting enWS's Gadgets for your needs).

Community-managed Gadgets need local community consensus and local Interface Admins to edit, so others (including the developers) can pretty much only offer advice.

PS. I see zhWS still has a lot of old code in Common.js etc. that was imported from mulWS/enWS. It would probably be a good idea to clean that up and modernise it a bit to avoid future problems and make site code easier to manage. enWS has now moved all code to Gadgets so Common.js is completely empty (we still have some ancient stuff in those Gadgets, but we're getting there).

Yes. We need a lot of tech workforce in zhws.