i can give you an example from my own experience, though it doesn't involve an actual extension, just a stamp in my passport.
back in feb i visited my husband and asked to stay until a certain day in march. the border agent actually gave me a few extra days (As they tend to do), and i found out 2 days later i had to go back home for a mandatory work related thing. i drove home and back in 2 days, and when i got to the border again, the agent saw i already had a future date stamped in my passport, so i just explained the situation and he let me in no problem.
while it's understandable each entry is it's own entry, if you already have permission to be there for a future date, and you had to leave for specific reasons related to your home ties, i don't think that is that big of a concern to CBSA. Again, as always, it's dependant on the border agent you encounter each time. perhaps it's different for extensions and if you've already been there long term, though i'd lean toward the belief if you already have permission to be there for a future date, they will allow you back in.