Reminder widgets have stopped showing their content.

Since your reminders still appear normally in the Reminders app, but the widgets show only grey placeholder lines, I would focus on the widgets themselves rather than changing your reminder data.

Start by restarting your Mac. 

Then open Edit Widgets, remove the existing Reminders widget, and add a new one. 

Before placing it on the desktop, check whether the new widget displays your reminders in the widget gallery. 

Apple’s instructions for managing widgets are here: Apple Support — Use widgets on Mac

If the new widget works in the gallery, add it to the desktop. 

You can then control-click and choose Edit Reminders to select the list you want it to display. 

Apple documents the Reminders widget settings here: Apple Support — Use Reminders widgets on Mac

If the new widget still shows only grey placeholder lines in the gallery, go to System Settings >> Desktop & Dock >> Widgets and make sure widgets are enabled on the desktop. 

Restart the Mac once more and test the newly created widget.

You can also restart the computer in Safe mode and see if the issue persists 

If the same problem continues in both the widget gallery and on the desktop, recreating the widget has not resolved it. 

I would leave your Reminders data and iCloud settings alone rather than making more invasive changes. 

Instead, check System Settings >> General >> Software Update for any newer macOS update. 

If there is no update available, the issue would be worth reporting to Apple, noting that the Reminders app displays your lists and tasks correctly while both the widget gallery and desktop widget show only grey placeholder lines.

 Product Feedback - and make it known to Apple regarding this ongoing issue

To fix the blank Reminders widgets showing grey placeholder lines after upgrading, you need to resolve an iCloud sync authorization block in your system settings. Even though the Reminders app itself loads cached data locally, major macOS updates often pause widget data streaming until background iCloud terms are accepted or account credentials are re-verified.

Follow these steps to restore your widgets:

Clear System Settings Badges & Terms

• Open the System Settings app on your Mac.
• Look at the top left of the sidebar for any red notification badges or action item alerts next to your Apple Account (your name).
• Click through any prompts asking you to Accept New iCloud Terms and Conditions or to Re-authenticate/Sign in to your Apple Account. 

Verify and Reset Reminders Sync

• In System Settings, click on your Apple Account (your name at the top of the sidebar).
• Select iCloud, then click the See All (or Show All) button under the "Apps Using iCloud" section.
• Locate Reminders and verify that the toggle is turned ON.
• Tip: If it is already on, toggle it OFF, wait 30 seconds, and toggle it back ON to force a refresh of the background syncing process.

Force-Restart the Widget & Notification Center

If the grey lines persist after updating your iCloud status, force the background widget simulator to restart:

• Open the Activity Monitor app (found via Spotlight or in Applications > Utilities).
• In the top-right search bar, type Notification Center.
• Select the NotificationCenter process, click the "X" button at the top of the window, and choose Force Quit.
• Do the same for the remindd process if it appears. The system will automatically relaunch them, which forces your widgets to rebuild their layouts and reload data.

There was a similar report on macOS 27.0:

Reminder's Widget become empty only fill … - Apple Community