What kind of battery did you put in ? A standard alkaline battery has different voltage discharge curve compared to the recommended rechargeable NiMH batteries (example)
You can’t really measure the capacity of a battery directly, all you can measure externally is the voltage, so the logic has to make some assumptions about the type of battery being used when calculating percentage. Since they recommend the LADDA rechargeable batteries, which are NiMH, they probably set the 100% threshold lower - perhaps as low as 1.2V, so as long as whatever battery you are using is above this voltage, it will show up as 100% - it might take longer for an Alkaline battery compared to NiMH rechargeable, but later you might have the opposite problem, when the reported percentage will start dropping faster for Alkaline vs NiMH, due to the different discharge curve characteristics.
More sophisticated devices can sometimes have a learning function, where they observe the voltage curve over a full discharge cycle of a battery, and then adjust the % reporting to more closely match, but since every battery can be different, this is still not always accurate. The discharge curve can be affected by temperature as well, further complicating the issue.
Since these sensors are supposed to last up to 15 months on single charge, and it actually wouldn’t be too unusual for a device like this to report 100% battery for up to the first 1/3 of its battery usable life (especially if using Alkaline batteries), 3 months is probably still within reason. There is often some intentional padding as well by manufacturers, as they don’t want customers to see big drop in battery percentage early on, as this might cause people to make ultimately incorrect predictions about total battery life, and complain or return the product.
MoCA is honestly really great. I’ve been running 4 MoCA nodes (2.0 and 2.5) in my house for 10 years now, over pretty shitty RG58 cabling and a bunch of splitters, quite long runs too, along with 1 gig DOCSIS 3.1 modem sharing the same cabling, and it has been absolutely bulletproof and reliable (especially compared to the powerline ethernet I was using before, which was absolutely terrible). Plenty of bandwidth for full duplex 1 gig - really the only downside compared to 1 gig ethernet is a few extra ms of latency.
Running ethernet cables everywhere would be very expensive in my situation, and while I considered it a few times to get more bandwidth between my main computer and the NAS which sits in my homelab in the basement, I still find it hard to justify the cost. If I were to pull the trigger now, I think I would go with fiber (or maybe both) - but I would have to replace so much stuff to get up to 10 gigs, all my homelab equipment is 1 gig only, server nics, NAS nics, switches, etc… it would not be cheap, beyond even just the cost of running the cables. So I am just sitting back and waiting to see how the technology evolves for now.