I suspect it's a sign of a memory issue, but I keep having a few hundred of my recent transfers changing their check number from "TXFR" to "TXFRúÿÿÿ".
I can do a search & replace, and it seems to fix it - until the next time I open Quicken.
I do have a very large quicken file (20 years worth of data, 350MB+). So, guessing that's a factor. But I would think after changing values back to TXFR it would stick - but it doesn't.