Battery health percentage not showing

There could be a couple things wrong with the phone.

I’m thinking that since that phone will be about 2 years old soon, that the battery may have been replaced either by that shop or before it was traded in. I would bring it back and ask what they can do about it. Sometimes cheap aftermarket batteries will cause issues like this.

I bought an iphone xsmax second hand from a shop. At the time of purchasing the battery health was 100% and the percentage was showing but once I got back home, I inserted my icloud and checked the battery health, it went into service and percentage was not showing. I’m really confused please answer my query, Thanks

My phone is not working battery health