It seems that the hyphen is present in the REST API response: https://pl.wikisource.org/api/rest_v1/page/html/Dziewczyna_bezimienna%2FCz%C4%99%C5%9B%C4%87_druga%2FXVIII (look for "doprawdy, że cza-"). On the other hand, the hyphen is not present in the Action API response (https://pl.wikisource.org/w/api.php?action=parse&format=json&page=Dziewczyna%20bezimienna%2FCz%C4%99%C5%9B%C4%87%20druga%2FXVIII&formatversion=2) nor in the normal read mode.

The description implies that the hyphen appears on regular page views (e.g. https://pl.wikisource.org/wiki/Dziewczyna_bezimienna/Cz%C4%99%C5%9B%C4%87_druga/XVIII), but I don't see it. I do see it on https://pl.wikisource.org/w/rest.php/v1/page/Dziewczyna_bezimienna%2FCz%C4%99%C5%9B%C4%87_druga%2FXVIII/html though (along with page numbers that were not in the regular view). I don't see the hyphen at https://pl.wikisource.org/w/api.php?action=parse&format=json&page=Dziewczyna%20bezimienna%2FCz%C4%99%C5%9B%C4%87%20druga%2FXVIII&formatversion=2 (but the page numbers are there).

Maybe there is some difference in parser options or hooks for the REST endpoint that makes the ProofReadPage tag hooks act a bit differently?

OK, so I can repro this issue with action=parse by using parser=parsoid, e.g. https://pl.wikisource.org/w/api.php?action=parse&format=json&page=Dziewczyna%20bezimienna%2FCz%C4%99%C5%9B%C4%87%20druga%2FXVIII&formatversion=2&parser=parsoid . The parse Action API default is the wikitext parser. The rest API uses the parsoid parser.

Sourcepage: https://pl.wikisource.org/w/index.php?title=Strona:PL_Dziewczyna_bezimienna_by_Ch_Mérouvel_from_Kurjer_Poranny_Y1893_No152_part07.jpg&action=edit

I believe ProofreadPage has a little known feature where a hyphen is removed when concatenated to the next page via https://wikisource.org/wiki/Wikisource:ProofreadPage#The_%3Cpages/%3E_tag
https://www.mediawiki.org/wiki/Extension:Proofread_Page#Join_hyphenated_words_across_pages

PS. There's also a hidden alternate separator in the text, which is https://www.mediawiki.org/wiki/Extension:Proofread_Page#Page_separator
You can see this in the Parsoid output because it has <span typeof=\"mw:Entity\" id=\"mwiw\"> </span> after the cza-, which is correct.

Maybe they are not well known beyond the Wikisource community. However for us these features are crucial. We use them almost everywhere.

In T411935#11466134, @Draco_flavus wrote:

Maybe they are not well known beyond the Wikisource community. However for us these features are crucial. We use them almost everywhere.

Yeah, but it's not a standard wikitext feature. it's something that is part of a specific parser tag, and it's easily accidentally overlooked if you are not deeply familiar with Proofread Page.

Change #1224259 had a related patch set uploaded (by Arlolra; author: Arlolra):

[mediawiki/extensions/ProofreadPage@master] Fix placeholder replacement in Parsoid renders

https://gerrit.wikimedia.org/r/1224259

Change #1224259 merged by jenkins-bot:

[mediawiki/extensions/ProofreadPage@master] Fix placeholder replacement in Parsoid renders

https://gerrit.wikimedia.org/r/1224259

Confirmed fixed, in both parsoid view and in epub export

Thanks, unfortunately the Parsoid native rendering was disabled in https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/1225613

The patch in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ProofreadPage/+/1224259 should have fixed the issue but we won't be able to confirm until it's re-enabled again.