Bug/Issue Steel Axe downgraded to basic axe while sleeping?

My son had already upgraded to steel axe in fall. On winter day 9 (I believe?)/ year 3, he chopped the log blocking the secret forest, went to ice festival, went to bed .... and woke up the next morning with only a basic axe in his inventory. I was watching him play and saw it happen. He had just gotten out of bed, said "why does my axe look different?", moved the cursor to it and found it was only a basic axe. How do we get the steel axe back? He's 7 yrs old and more than a little upset after all his work to upgrade it. Thank you for your help.