Assuming you have "Retain tabs from previous session" set at the URL opera://settings/onStartup, it's not normal for you to be losing tabs (when exiting Opera and starting it back up or when updating).
It does happen to a few, select users though. As to why, no one knows.
Could be corrupted session files. You can goto the URL opera://about, take note of the "profile" path, close Opera and delete the "Sessions" folder in the profile folder. You'll lose all tabs again, but that might fix it from now on. No guarantees though. Before you try it, you can right-click on a tab, goto "save" and choose "all tabs to a speed dial folder". Repeat for each workspace and window. Then, after you wipe your Sessions folder, you can right-click those folders and choose "open all in tabs" in the corresponding workspace and window etc.
Of course, that doesn't explain why it only happens after an update though unless, but maybe it will help anyway.
It could be something with the permissions of your profile folder that throws off the update for some reason.
If you're letting a program like Kaspsersky update Opera for you, that could be the cause.
If you get Opera via Chocolately and it updates Opera, maybe it doesn't update Opera right.
If you get Opera via Winget via its official repository or another one, maybe it doesn't update Opera correctly (as in, doesn't use the right arguments for the installer or something).
Maybe you have some old Opera Update tasks in the Windows Task Scheduler. You could uninstall Opera (don't choose to delete your data when uninstalling) and delete everything in "C:\Windows\temp" and "C:\Users\yourusername\AppData\Local\temp". Then, delete Opera's install folder (path at the URL opera://about). Then, open the Windows Task Scheduler, select "Task Scheduler Library" and delete any Opera tasks that remain.
Then, download Opera and reinstall it. Make sure to click "options" in the installer to review and adjust all the options as you like.
Then, maybe things will work.
Or, you might have to start from scratch by deleting the "Opera Software" folders in "C:\Users\yourusrename\AppData\Roaming" and "C:\Users\yourusername\AppData\Local" to fix permissions etc.
If you've ever right-clicked on Opera's shortcut and choose "run as administrator" to run Opera, that might mess things up.
Another thing it could be is anti-virus interfering with Opera's updater. What anti-virus do you have?
You could also check the health of your hard drive with CrystalDiskInfo.
You can also check your file system. See the "How to Run CHKDSK With the Right Flags" section at https://www.makeuseof.com/tag/stuck-chkdsk-use-fix-right-way/.
Then, you can check your system files.
It might be necessary to wipe out Windows and reinstall it.
Or, it could be a bug in Opera's updater and or installer that no one knows how to manually trigger.