Engine light, possible misfire?

Hello, a flashing check engine light is usually a Catalyst damaging misfire, Hence the flashing to get your attention. You have pretty low mileage, and it does seem to be an odd coincidence. I'd be curious to know what codes are stored to begin with and on a full system scan of the vehicle, not just the engine computer (ECM/PCM).
How many days was it and how far did you drive it before the light came on the first time after the coolant flush? I would be questioning exactly what they did. Did they have to move certain sections of the wiring harness and why did they replace the thermostat, what codes were stored the first time you went there? (that should all be on the repair order they gave you after the first repair). And a list of the codes stored this time. Really you should be able to call right now, and they should be able to tell you what was stored for codes, it only takes a few minutes to scan a vehicle for trouble codes.
Did they get their diagnosis wrong the first time? I'll take a look at the wiring diagrams for this vehicle and see what sections of the thermostat might be electronic for a 2017.
The vehicle wasn't overheating the first time, was it? You just went for a check engine light?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Here are some pretty interesting service info on how the engine computer (PCM) keeps track of the thermostat operation. If this is the code that set, and it came back after they replaced the thermostat, they might be confused on its operation. In this case even an Ambient Temperature sensor that's out of range could cause the code to come back because certain conditions are not met.
There is also a Technical Service Bulletin on a software update for the PCM due to some certain codes setting, as well as some idle issues and also aid in diagnosing throttle body issues. PCM software updates are pretty normal, every vehicle will have software updates that are programmed if a module is replaced, or if you happen to have the vehicle at a dealership. If you're within your warranty still, I would consider getting the PCM updated. There might be codes setting that are not actual faults. (diagrams 4-6 below)

They diagnosed a failed fuel injector and replaced it. They said it was unrelated to the prior repair. Roughly 6 days and 300 miles had passed between the thermostat repair and the engine misfire. The vehicle did not overheat either time, to my knowledge. Seems like an odd coincidence.

What were the original codes the first time you brought it there? Unfortunately, some shops are not honest. I have seen ignition coils that were located under the upper intake manifold with the coil harness plug just sitting there, never clipped and locked into the ignition coil and eventually the coil's plug falls out and a misfire code sets. It's possible for these types of faults to occur, but you only have 65k on the vehicle. I highly doubt an injector failed like that. Especially since she was able to drive it home with no issues after a restart. Did they list the codes on the first repair order? Sorry this type of thing really bothers me.

I just looked at the original receipt and they did not list any codes. However, before we took it to Firestone, we had AutoZone run the codes and it said Thermostat. On the new invoice for the Injector, again no codes listed.

Yeah, that's strange they should have listed all the code numbers, that way we can look up the code setting criteria to determine exactly why it ser. A thermostat code on this vehicle is related to the engine computer monitoring the engine coolant temperature sensor, and a specific amount of time have gone by. Next time you have any issues at all, have AutoZone or whatever auto parts store you use scan the vehicle for codes and come here and post them so we can give a list of possibilities ahead of time. So, you know what to expect and not just have to take a shop's word. Because they can say anything and then charge people anything as well and labor rates are extremely high these days. I question the injector issue, especially with this vehicle, the injectors might be under the intake manifold depending on the design and that will just add to the cost if it needs to be removed to get to the injectors.
I'm sure it wasn't cheap. And in reality, replacing one injector can cause issues because different manufacturers or an aftermarket injector will have a different flow rate that the other ones still installed in the vehicle. But come to us next time first. It doesn't cost you anything, and that way you can go to a shop prepared better.