You're ok. People are getting too touchy about this. If the devs didn't want you to add stuff to your reading list, they should've kept that function shut off. I do recommend waiting a little longer to see if the lists fix themselves, but if you don't mind the order being different from before, adding things back is probably fine.
If you don't want to touch your reading lists, you can use the mgg bookmark function. As far as I can tell, mgg bookmarks are working perfectly fine and have not been affected by the reading list problem.
To bookmark a chapter/page on mobile, open the chapter/page you want to bookmark and hit the button at the upper right corner with three horizontal lines. A dropdown menu should open and you should see something that says "bookmark it". Hitting that button will bookmark that chapter for you. On desktop, there will be a button on each chapter/page on the upper righthand corner with a heart and text that says "bookmark it".
To find your bookmarks on mobile, get out of the manga, click on the button with 3 horizontal lines. You should see a text in the menu that says "bookmark". On desktop, you can click on your pfp on the upper righthand corner and you'll see a "bookmark" option on the menu. You can also see what pages you have bookmarked if you go to that manga's homepage. So if I have chapter 24 bookmarked on Our Sunny Days, I can go to the Our Sunny Days manga page and it will show that I have chapter 24 bookmarked.
Ah, I forgot to mention that you don't get update notifications for manga you "bookmarked" and I believe there's also a limit to how many bookmarks you can have on mgg but I don't remember the number. So that kinda sucks but bookmarks should help you keep track of what you're reading until they get lists corrected.

I like to apologize to the devs for my idiocy for adding more work for them when they already swarm with works. I hope everyone get enough rest when working on this glitch.
Again I'm so sorry for adding manga to 'want to read' 'Reading' and 'Already read'