My cousin was an O1 in the Army and he was medically discharged after he had a heat stroke while AD. He had no history at all regarding heat stroke, this is a guy that played FB throughout HS and in college in 90+ degree weather in full pads, but being in Iraq during the summer and wearing all that gear while in a Humvee can cause heat stroke regardless of how much water he drank. They sent him back to the states and placed him through medical tests, needless to say the Army decided they needed to part ways.
I am assuming you are not on scholarship, because you would have to mention this on your DoDMERB intake questionnaire and that is where you would place that you have a history of heat stroke. From there they will determine if the history merits a qual, remedial, a disqual. If it is the latter two than you will go through the process of being waived or denied. I don't know for other ROTC branches, but AFROTC will require a DoDMERB exam prior to commissioning even if you were never on scholarship. Thus, somewhere along the line it will come up. Just get your medical records in order and have an explanation of why it occurred.