This is very strange. I don't have an explanation. If you would like to investigate further, there are a couple things you could check:
Open /Users/tkh/Library/Arduino15/package_index.json in a text editor and check if it contains the following text:
{
"name": "Arduino AVR Boards",
"architecture": "avr",
"version": "1.8.2",
"category": "Arduino",
"help": {
"online": "http://www.arduino.cc/en/Reference/HomePage"
},
"url": "http://downloads.arduino.cc/cores/avr-1.8.2.tar.bz2",
"archiveFileName": "avr-1.8.2.tar.bz2",
"checksum": "SHA-256:6213d41c6e91a75ac931527da5b10f2dbe0140c8cc1dd41b06cd4e78b943f41b",
"size": "4918242",
"boards": [
{"name": "Arduino Yún"},
{"name": "Arduino Uno"},
{"name": "Arduino Uno WiFi"},
{"name": "Arduino Diecimila"},
{"name": "Arduino Nano"},
{"name": "Arduino Mega"},
{"name": "Arduino MegaADK"},
{"name": "Arduino Leonardo"},
{"name": "Arduino Leonardo Ethernet"},
{"name": "Arduino Micro"},
{"name": "Arduino Esplora"},
{"name": "Arduino Mini"},
{"name": "Arduino Ethernet"},
{"name": "Arduino Fio"},
{"name": "Arduino BT"},
{"name": "Arduino LilyPadUSB"},
{"name": "Arduino Lilypad"},
{"name": "Arduino Pro"},
{"name": "Arduino ATMegaNG"},
{"name": "Arduino Robot Control"},
{"name": "Arduino Robot Motor"},
{"name": "Arduino Gemma"},
{"name": "Adafruit Circuit Playground"},
{"name": "Arduino Yún Mini"},
{"name": "Arduino Industrial 101"},
{"name": "Linino One"}
],
"toolsDependencies": [
{
"packager": "arduino",
"name": "avr-gcc",
"version": "7.3.0-atmel3.6.1-arduino5"
},
{
"packager": "arduino",
"name": "avrdude",
"version": "6.3.0-arduino17"
},
{
"packager": "arduino",
"name": "arduinoOTA",
"version": "1.3.0"
}
]
},
Check /Users/tkh/Library/Arduino15/application.log to see if it contains anything that looks relevant for the time frame when you last opened Boards Manager.
If you are just interested in a workaround to get you using Arduino AVR Boards 1.8.2, you should be able to do it by deleting /Users/tkh/Library/Arduino15/packages/arduino/hardware/avr/1.8.1. Please be very careful when deleting things on your computer. When in doubt, back up! Arduino IDE 1.8.12 comes with a bundled copy of Arduino AVR Boards 1.8.2, so deleting the installed Arduino AVR Boards 1.8.1 will force the bundled version to be used.