Battery Life Changed

Myself and an Apple online support person ran this diagnostic program and no issues came up. I was told to just pay and replace the battery. But I felt this was not fair. After upgrading to Big Slur this happened. The laptop is 2017 so out of warranty, have been told it will be about £100+ to replace the battery even though the diagnostics show there is no problem with the battery. The person online said there was nothing they could do and it would not be covered even though it happened after the upgrade. I just feel this is unfair from Apple. I have been a loyal customer for many years. After the upgrade it wiped my hard disk which the online support person said yes sometimes this happens, (I did backup before) it should not have happened and I feel now Apple just want to wash there hands and charge me to fix a problem that cannot be solved with their diagnostic software, so they just replace the battery charge me and hope the problem goes away.

Made no difference. Ran the SMC correctly according to the documents online. Battery still saying.......Service Recommended....