I watch ~400 tickers and they all are missing data on 2021-03-11 and 2021-03-17. Here is a curl cmd to help recreate the issue:
$ curl -X GET \
-H "APCA-API-KEY-ID: ${YOUR_API_KEY_ID}" \
-H "APCA-API-SECRET-KEY: ${YOUR_API_SECRET_KEY}" \
"https://data.alpaca.markets/v2/stocks/PLTR/bars?adjustment=raw&start=2021-03-11T00:00:00-05:00&end=2021-03-12T00:00:00-05:00&limit=10000&page_token=&timeframe=1Min"
Response:
{"bars":[],"symbol":"PLTR","next_page_token":null}