New battery needing a ‘service’

Hello, I’ve just replaced my battery for a second time ( first time phone would randomly turn off) and second time I’ve just got my I fix it battery and now the new iPhone update says ‘service’ in the new battery health check option. With the previous battery it said all good.

Now this newest battery seems to be working fine after two days and the shutdown issue hasn’t happened yet. So, can anyone advise why this comes up from Apple? I must admit I didn’t fully charge the phone the first time I installed the battery but have since fully charged and allowed to go down to 10% before charging again

Thanks

Matt