Reader mode button doesn't show up on Plus Minus articles on rp.pl

Originally filed at: https://bugzilla.mozilla.org/show_bug.cgi?id=1965770

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:138.0) Gecko/20100101 Firefox/138.0
Steps to reproduce:

open any article from Plus Minus section of rp.pl (https://www.rp.pl/plus-minus/), eg. https://www.rp.pl/plus-minus/art42248991-jan-oldakowski-co-dzieci-pamietaja-z-czasow-wojny-chcemy-ocalic-te-wspomnienia

Actual results:

There is no reader button

Expected results:

There should be a button.

---- more info
This is mostly visible and quite annoying on Firefox mobile where it would be most welcomed.

I think the issue is that rp.pl has really, really odd way of handling user session so it quite often (always?) load paywall page and then detects cookies and reloads the page, but then the button is missing… And PlusMinux columns are quite lenghy so having more readable view (as well as reading time estimate) would be very welcomed.

I tried setting reader.parse-on-load.enabled in about:config (not available or mobile) or using https://addons.mozilla.org/en-US/firefox/addon/activate-reader-view/ but withough much luck.

It would be great to have an option to force-dispay the button always…

Start by reproducing the issue with the linked Plus Minus article on rp.pl, including the paywall and cookie-driven reload described in the report. Determine whether the missing reader button is caused by Readability or Firefox's reader-mode integration; done means identifying the responsible component and defining a verified fix for the reload path.