IPhone XS Max Battery Health Reading Error

Hi

I bought an iPhone XS Max a couple of days ago on eBay. The batter reading was 98%. This morning when I checked it the reading had a dash where the percentage was and a message letting me know that the battery was significantly degraded. I did a restart, forced restart and a reset Network Settings. The problem persists. I don’t think it could have had a after market battery replacement as I got a 98% reading the first two days. The phone is also in mint condition without a scratch on it anywhere so it doesn’t seem to have been used.

Can anyone help with this problem please.