I started to receive this mysterious missing events on my Events Manager and when I go to the page (https://www.fonirra.com/products/t3?variant=S713210490242732039), I found all pixel request sent to Facebook has a proper event name. What's more curious is that the URL source shows fbapp://350685531728/unknown which is not the page I am advertising. What are these "missing events"?
It looks like it could be caused by the img tag anchoring (such as <img src="https://www.facebook.com/tr/?id=311573544324909&ev=ViewContent&cd%5Bcurrency%5D=USD&cd%5Bcontent_ids%5D=G677764488308461568&cd%5Bcontent_type%5D=product&cd%5Bcontent_name%5D=Hi%20Vis%20Safety%20T-Shirt%20Reflective%20with%20Pocket%20High%20Visibility%20Work%20Shirt&cd%5Bcontents%5D=%5B%7B%22id%22:%22G677764488308461568%22,%22quantity%22:1%7D%5D&cd%5Bvalue%5D=29.99&eid=fc22bde0-dde4-11ec-44ab-cd28cb4347fe&ud%5Bexternal_id%5D=b250d02aff979831efc78247b217dbcf2a8ff96ef48e4e277f7f83cd89ae05ce" height="1" width="1" style="display:none"/>) we added on the website.