What generally happens here is that the cheaper seat is in a nearly exhausted fare bucket (e.g. I1 ) and the booker (you), in selecting it "holds" the seat reducing the fare bucket availability to zero (e.g. I0). When you exit and come back in, depending on how you exit and how the airline software's handles that exit, those seats can remain "held" for a period of time. As a result, a search immediately after your exiting returns availability in a higher fare bucket (e.g. D) as the systems see none left available in the lower fare bucket. To the booker, this can make it seem the cheaper seats have been sold. Later on, the holding times out and the cheaper seats get returned making the cheaper fare again available.
Similar has happened to me in the past. In such cases, if I need to recheck times etc. such as what happened here, I avoid going back and use a different resource/browser to make that check.