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2016 Canada arrival having passport stamp but 2018 doesn't. any idea?

Limited info here but ... When did you become a PR? PRs don’t tend to get stamped (never even needed passport since PR card)
 
2016 Canada arrival having passport stamp but 2018 doesn't. any idea?
If I understand your question clearly. As a visitor in 2016, passport was stamped on entry. While in 2018 wasn't stamp. Yes, it is possible. There are new self check-in kiosk in some international airports in Canada. If you came in through any of those, your passport will be e-stamp not physical. However, if on the same spot you need a stamp on the passport, you need to approach a BSO of CBSA to do that.
 
If I understand your question clearly. As a visitor in 2016, passport was stamped on entry. While in 2018 wasn't stamp. Yes, it is possible. There are new self check-in kiosk in some international airports in Canada. If you came in through any of those, your passport will be e-stamp not physical. However, if on the same spot you need a stamp on the passport, you need to approach a BSO of CBSA to do that.

In 2016 i landed first time on PR and passport was stamped. Any idea since which year Kiosk became operational?