When I shut down my feeder for extended periods, they show up in My ADS-B stats as grey bricks. Today, my ISP cut off my line for close to 2 hours, but there is no grey brick.
The clue is in what you said. “Today, my ISP cut off my line for close to 2 hours“.
From what I have seen, the grey brick only appears if there is NO data received by FlightAware in that hour. Your connection probably went down a few minutes after the hour so there was some data received for that hour. The connection was the restored a few minutes before the second hour so again there was some data received in the second hour.
If you hover or tap (I’m not at my computer at the moment to confirm which it is) your pointer over the blocks it will tell you how much data was received during that hour.
You are correct. My downtime was from 20:23 to 21:43 UTC - FlightRadar 24 has an “availability” chart. There is not a whole missing calendar hour. If I hover to reveal numbers, I can sense a data dip during those two affected hours. It’s just that flight pattern on the day made the dip less obvious without examining numbers.