Given your description, your Tab Groups may have been lost or deleted for reasons unknown. This is certainly not a frequently reported issue - or one that I have experienced over the lifetime of iPadOS 18/26 updates.
You may be able to resolve the problem by deleting a single cookie from your Safari cache - this, surprisingly, being an internal cache/database that can occasionally become corrupted. From iPad settings:
Settings > Apps> Safari > Advanced > Website Data >
Allow the list to populate (this may take several minutes) - then, using the Search box at top-right, search for "cdn-apple.com" (without the quotes). When located, delete just this entry (swipe to the left to expose the associated delete button).
If the Safari App is still running, force-close the App from the App Switcher / Exposé window - and restart Safari.
Removing the cookie forces Safari to refresh its local store of saved Tabs, Tab Groups, iCloud Tabs, website icons etc. If your saved Tab Groups have not been deleted, this surgical intervention may recover them. If not, they are likely lost and unrecoverable.