I noticed a drop recently also. I was getting ~ 45-46 in 2018 Touring model from late May 2018 when I got it. I got almost 60 on the few longer trips I did. The Touring being heavier is is rated lower than other models but I figured I was doing well, I think using cruise control helps a lot even in 25 mph zones.
I had the recent BMS update and then immediately noticed a drop to ~ 42 mpg. This was before the weather cooled off. I ran about 1/4 tank then filled up again. Now the weather has turned colder and I am at 40 mpg for the 3/4 of that tank I used. Generally though I use Shell gas although from different stations. I also noticed the change in battery charge level, previously varied from maybe 3/8 to 5/8 full. After the update it varies from maybe 1/2 to 3/4 full. I don't se this changing fuel consumption though directly. The engine does start up much sooner though (even in the garage)but I think this is to warm the car as I have heating on and it will need to run the engine to warm the air and use more electricity to warm heated seats. This too will increase the fuel usage in the winter.
I checked with the dealer service dept. and they said to try different gas stations to see if that made a difference based on blend. It will take me a while to do that at my mile consumption. I do generally low mileage (under 4000 so far in 6 months) so do not get through many tanks of gas to compare gas blend changes but I will try that anyway. I did check the tires, they are all at 35 (cold weather drop I expect) - another lb in them would help a minor amount.
I think maybe I got triple played here. BMS update, winter gas blend and cold weather.
Despite that though I still really like the car. I was talking to someone who got a Hyundai 2018 hybrid and they also have the smart cruise control although his will work in stop and go driving to a dead stop so it works for highway stop & go with no intervention. I wish the Niro would do that - braking to 5 mph is nice but to a dead stop and starting up again would be a lot better.
2018 Red Niro Hybrid Touring - Northern Virginia USA.