[kanawa]

They don't break down in septic tanks

I've heard the same said of Kirkland toilet paper (Costco).
Know of any basis for that claim?

[Sergio]

As to the basis for the claim, it would be anecdotal evidence from folks who have septic systems and have had issues. I did a quick search online and found this:

https://thetoiletzone.com/flushable-wipes/

Hope this helps.