The receiver reselection happens every ~30 seconds. The actual data forwarding is continuous once you’ve been selected.
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I experienced occasional loss of MLAT targets on Piaware some time ago and searched for the reason on here. Found Oliver’s explanation at the time which was honest (and repeated again here).
But seems like we are in a Goldilock’s situation:" too few" receivers = no MLAT, “too many” receivers = we might get MLAT from time to time. We hope to be in the “just right” situation where FlightAware needs all of the data to provide any MLAT and hence all the data comes back. Probably considered a “1st world problem” 
That is one of the reasons I run another receiver into PlanePlotter as an alternative for MLAT when the target vanishes from Piaware.
However the comment “Anyway you didn’t pay flightaware, you bought hardware to use their system.” is a bit misleading. If we did not supply data, FlightAware would have nothing to sell on, so it is a mutual benefit situation.
Finally, playing devil’s advocate, what happens with the increase in ADSB installations? No need for MLAT any more, no need to be connected to the internet, just use dump1090 without PiAware? Not quite that simple given there will not be 100% ADSB - just a thought.
My receiver is synchronised with 470 other receivers for mlat. Relatively close by is an international and a GA airport, which is mainly used om sunny weekends.
So even though there are many receivers, my impressions is get all the local mlat traffic, cannot really remember hearing or seeing a plane flying by without being tracked. That said, it is mostly the GA planes and some Dash 8s that do not seem to have ADS-B.
What I discovered is that planes further away have gaps in their mlat tracking, so that will be due to the way the receivers are chosen.
I wonder if they are counted in both categories, other and mlat?