Does anyone has problem with battery reading on iOS version 14.6 with older phone?

I have bought a new iphone 12 pro and I am still using iphone 7. iphone 7 which is perfectly fine performance wise but, it seems to have problem after the new iOS update. The phone is working alright, but the trouble is with the battery reading. I thought, it was the virus infected in my phone but apple does not seem to have antivirus. The phone battery reading would be 23 per cent and moment later shoots up to 67 per cent. When I reboot the iPhone, the battery level went up to 89. It goes the downtrend too, while doing the reboot and at one point it shows needs to be charged even the phone was 96 per cent moment before reboot. Any one is having such issues with your older phone with the new iOS. Has anyone complained the ACCC in this matter. This is happening these days more frequently.