There's a lot to unwrap here, and the short answer is to ignore the heart rate and estimate your base run pace by perceived effort (breathing, ability to hold a conversation during the run).

Much of the confusion comes from defining your heart rate zones. There are three common methods: one is based on your max HR, the second on your heart rate reserve (max HR minus resting HR), and the third one on your lactate threshold rate (at which HR you cross from aerobic to anaerobic pace). The first two methods have the obvious drawback that the max HR is often mis-estimated. For this reason I prefer the lactate threshold HR, which my Garmin estimated very accurately. To make matters even more confusing, various literature uses different percentage ranges for HR zones. But wait, that's not the end to confusion. Garmin seems to categorize your runs by your pace, and not by your HR zone. So, for example, when I run my recovery run at 5:50/km and fully in Z1, Garmin tells me I had a base run. But when I run it at 6:30/km in a 50-50 split between Z2 and Z3 due to hills, Garmin puts me into recovery.

So, I wouldn't put too much stock into your watch and instead just focus on how you feel. If you run with someone, talk to them. If you are alone, occasionally talk to yourself. If you are able to talk normally, you are running in your low HR zone.