I don’t think this has anything to do with tariffs.
A quick search shows there have been continuing shortages over the years.
Here’s one from 2012 on the old Model Railroader Forum:
Jul 2012
"I had 8 cases of it on back order since about January of this year. I called Midwest almost every month and was told it was a result of cork shortages. They expected a shipment by summer and I finally received my shipment a few weeks ago. "
There are hundreds of posts over the past years about why there is a shortage of cork roadbed, especially centering on Midwest shortages.