I can’t think of a reason off the top of my head, either. But it made for a good minimal working example of the behavior I see.
It looks like you need to provide an upper bound, otherwise it takes a default (which appears to be today).
https://data.alpaca.markets/v1/bars/1Min?symbols=AAPL&limit=1&start=2020-05-12T12:30:00-04:00&end=2020-05-12T12:30:00-04:00