It isn’t documented besides this single case, so not common.
Maybe there was a technical issue with his Fireglo during the from the Basement sessions, and so he strung the Jetglo to six strings. Or maybe it was just on a whim; he wanted the sound of that specific Rick but felt like using six-strings for Staircase. Then, when it came to touring, he went back to twelve-strings for Airbag and just kept it for Staircase.