The mushroom is totally healthy, but you are right, it's due to dehydration.
Actually it's common case for many mushroom species
Mushroom block may be moist enough, but the air outside is too dry.
Mushrooms consist 90% of water and require relative humidity (RH) about ~90% for pinning and fruit bodies development as well, when room RH vary from 30% to 70%.
So it's sharp difference! As a result, mushrooms lose moisture extremely fast. That's why these splits appeared.
To maintain high RH (for home growing) cultivators use simple fruiting chambers - plastic tub 40-80Qt with manual misting (aerosol misting bottles are great to use) or terrarium humidifier + fresh air exchange with air pump, fan, or manual fanning (mushrooms breath and also need oxygen)
Or grow tent with manual misting (aerosol misting bottles) or terrarium humidifier/fogger
To check RH just use simple hygrometer
Have a happy growing!