I've had my cookies get slightly soft in the dehydrator on a few occasions - usually when the cookies were large, had a lot of icing on them, and were in the dehydrator for a relatively long time. (If you were to stick cookies back into the oven, they'd typically soften too under the heat.) BUT, I am careful not to move large cookies from their racks right away; rather I remove the whole rack from the dehydrator and allow the cookies to cool on it before I move them. The cookies will usually firm right back up.
As for rippling, that too has happened to me on occasion, both within and without a dehydrator. When it's happened in the dehydrator, it's again usually been with very large cookies with lots of flooding icing on them OR if I put the cookies too close to the central channel (fan area) on my Excalibur machine. I find that if I push them as far away from center as possible and crank the temp to closer to 105F, the icing will set more quickly without any wave.
I also like to push my icing consistency as thick as possible, because thicker icing will also set faster, usually before the cookie can get soft and without any rippling.