I am worried about it too. I requested to get travel report history. I do not know how should I do.
My trip to the US via the same bridge got me a stamp.
I find the business of exit records interesting. When I renewed my PR card five years ago I made a request to CBSA asking for entry and exit records for the previous five years. CBSA responded with all my entry records but pointed out that Canada does not collect exit information. Maybe that changed.
Still not convinced this is the right chain, however on the CBSA part correct Canada doesn't collect exit information.
When you fly out of Canada, there is no CBSA checkpoint / exit area. Same as the UK and US.
However many maybe even most other countries, when you depart you'll pass through a kiosk where they check / scan and in many cases stamp your passport. This happens all around Europe and all countries I've been to in Asia, Africa and South America. Plus both New Zealand and Australia. I've gotten two Australian stamps before on a two hour transit (I believe they, like NZ+UK and possibly others, no longer stamp eligible passports due to automated control gates).
I've written the stamps for both entry and exit (where present) into my spreadsheet ready for doc check. Most countries make it easy and stamp both on the same page, but some do not. The best is Russia where they line the stamps up start at the top of the page and working down. The worst country for stamps is the US (entry only) where they'll stamp randomly over the top of other stamps, upside down etc and they also fade far faster than all others!
Just complete your history in full, while it's annoying we can't be held responsible for other countries or even CBSA's occasional refusal to stamp our passport. On an EU passport I've even asked (and been told no) when asking for a stamp before. But I still put the entry in.
And as a matter of point I got stamps (and I-94 as I've visa exempt and they ignored my ESTA - written in pen) when I last entered the US via rainbow bridge.