[Maljunulo]

I know nothing of Sprint's coverage in CT.

Are there areas in the state where Google Fi will switch from T-Mobile to Sprint?

I haven't found any locally, but I have only been on Fi for two days.

So far so good, after a very long time as an AT&T customer.

Pixel 2 in Tolland County.

[Rukbat]

It will, if the TMobile signal disappears and there's a Sprint signal.

[Maljunulo]

My phone switched to Sprint this afternoon when I visited a Walgreens that is pretty much a T-Mobile dead spot.
The CDMA side is useable, but the LTE side is pretty bad. The signal hovers around -120 dBm, but it varies fron a little better to a little worse, and even dropped for a while. I'm assuming that the fact that the other side is useable kept it from switching back to T-Mobile.

I tried a restart and it stubbornly stayed on Sprint.

I'm also assuming (hoping) that when I go out tomorrow it will switch back at some point.

Fi needs a "Reset Carrier" button, that will send it off looking for the best signal, rather than waiting for it to make up its own mind to switch.