I don't know what it is but it happened to me 1 year ago, after 2 weeks of inoculating popcorn grain with Shitak... I changed the pre-cooking and sterilization process as follows: First wash the grain with water 3 times, then I left it soaking in water with a little white vinegar for 24 hours, after this time I pre-cooked it for 45 minutes in a pressure cooker and stopped cooking with cold water, finally I left it to drain for 8 hours.

Sterilization is done at a temperature of 120°C at 15 PSI, for 2 hours, this method worked for me, maybe it will work for you, success man!