It appears fundamental data might be lost for (some) delisted stocks.

I've been working on an algorithm that looks at fundamentals, a few weeks ago I saw the stock CTEK was getting some hits in my code occasionally so put in some extra log messages to track it.

CTEK was delisted on Sept 1st, and after coming back to my algorithm and running it again I now see that it is never being detected.  Some debugging is showing me that fundamental data isn't available any more.

Has anyone else noticed anything like this?  Is it a widespread occurrence?  It implies a fairly worrying survivorship bias problem in any algorithm based on fundamentals.