Pace tracking waaaay off.

Watch pacing threw me off today so bad I skipped my optional easy run. 10 weeks into my plan with coach Greg, I pretty much know what my zone 3 feels like. Especially coming off the back of my long easy run on Sunday where I logged 9.19km over 50mins of actual run with an average heart rate of 140bpm and average pace of 5:27/km, I think I've earned the right to know that something was really off about the pace Garmin registered today. I watched my heart rate climb to 152 and my watch was telling me I was doing 6:xx/km. Occasionally, it would tell me I was within desired zone then Ill hear 6:xx/km again. I use a chest strap so I kept an eye on my heart rate on the treadmill. The effort was definitely inching towards zone 4 but that effort, HR and 6:xx/km was definitely an error. Has anyone experienced this before?