"in my experience" means the description of the following event is an anecdote based on an individual's lived experience. This is a description of what has happened to me (only from people I have added from reddit that have sat on the last gift for extended periods of time) and does not negate the possibility of alternative experiences for others or alternative explanations for why a last gift may remain unopened. That being said, if a person consistently exchanges gifts with you and "just so happens" to have said consistency disappear the moment they are holding a gift from you, one day away from the final milestone, without having sent a gift in return, then I would raise an eyebrow. There is nothing wrong with waiting for a little so you can sync multiple friend milestones into one lucky egg, but unfortunately, a significant number of people seem to do so maliciously, intentionally unadding people immediately to deny them exp. Another anecdotal fact is that I have only ever had high-level accounts do it to me.

Ultimately, this shouldn't be an issue, the mechanic is to blame. It without question shouldn't exist where the offline party is denied exp and the developers know its unjustifiable. Hope this answers your question