This is a FB paper issue, nothing to do with over washing RC papers. It's exactly what I had happen when I switched to this particular company's tissue. Too hot/too long and it does this, it's getting the balance right.
With the older shellac tissues there was no problem but then they need temperatures that can cause major problems with RC papers so newer tissues were developed. Get these too hot and they soak into the mounting card, as the card re-adsorbs moisture the ridge becomes aparent.