The task description is still not explaining what the actual results and expected results are... For future reference, "do not work" or "worked fine" is not enough information as it does not describe what happens and what should happen instead. Anyway:

Steps to reproduce:

• I went to https://fr.wikipedia.org/wiki/Spécial:Préférences#mw-prefsection-gadgets and enabled all gadgets.
• Then went to https://fr.wikipedia.org/wiki/Spécial:Préférences#mw-prefsection-betafeatures and enabled all beta features.
• Then I went to some random pages via the "Article au hasard" link.

Expected results:
Consistent displaying of either the local gadget or the beta feature when putting the mouse pointer over links
Actual results:
Only on some articles the gadget popup is displayed when putting the mouse pointer over links:

Ping @ovasileva

I'm not sure I understand the problem here.
Popups extension is currently disabled on Wikisource (see T147553).
On French Wikipedia the gadget if enabled will override the Popups extension.

Everything appears to be behaving as expected here.

Today, and waiting 20 to 30 seconds after activation in my case, the Popups gadget or the feature run as expected in wikipedia and wikisource. This delay explains perhaps my tests results and depend probably of the charge of servers.

The "page preview" unavailable in wikisource is the edit panel with a button to clic to try some changes. Wikisource replace that by a dedicated comparator with the picture of each page of a book in the namespace Page:

The namespace Page: is not available for simple readers of books. Then it cannot disturb them.
Also the Popups gadget and the feature are usefull for correctors who correct or verify hundreds pages each day and discuss about their works and look at their watchlists.

Then the Popups gadget and the feature are a treasure to preserve the efficiency of admins and correctors, without bother simple readers.