There's been countless thread highligting this issue for Garmin Watches.
Clearly a software issue. The timer/stopwatch isn't putting the watch into idle mode, even if the app isn't actively counting anymore.
https://old.reddit.com/r/Garmin/comments/rkl1n0/f6_pro_stopwatch_battery_drain/
" It's the code in this countdown function. Like what previous information stated before, it doesn't put this countdown function into idle or stall state. Instead, it kept running this code even count down process isn't even started or doing its function. The point here is if count down is "doing" the count down, it's normal to burn the power. If it's not doing any countdown, this process needs to go into stall state. The problem here is it will continuously burn the power at the same rate no matter it's counting down or not. This is very bad behavior. If I forgot to exit this count down mode/app, it will burn the battery crazily like I am doing endless count down. So, this function needs to be fixed. "