It seems unlikely that the upgrade for Linux Mint was to blame. It seems more likely to be unlucky timing. So you might want to check an alternative distribution first to convince yourself of it being hardware or software.
If you have a USB stick available you could try a Live USB to boot an alternative distribution (eg Ubuntu) to see if it behaves differently. And even try an older version to ensure that it’s not a new kernel bug.
I spent 5 years managing Lithium Ion batteries used as power plants on the UK power grid. While consumer grade laptop batteries are a bit different, one thing is still true: measuring battery health and state of charge are a dark art and liable to be wrong for any number of reasons. But state of charge is certain when the battery is full or empty.
In laptop batteries, a battery that won’t charge more than 60% is a near certain sign of a bad battery, and there are not really alternative options but to replace the battery.
The fact that it reports “excellent” health may just be that the measuring algorithm can’t detect this specific fault. These algorithms usually rely on measuring charge in/out against state of charge changes. But some faults fool the state of charge measurement which are amazingly flaky to begin with. So fooling SOC also means fooling battery health.