@Ryan *******
, It's not an actual "overstay". But the Thai immigration would treat it same as overstaying if you don't cancel some extensions for specific reasons such as employment, studying, etc.
If you didn't cancel those extensions, not only you may be fined for overstaying, but also you may not be able to apply for another long-term extension in your future.
Sadly, it doesn't matter if that makes sense or no sense to foreigners. It is how Thai IOs handle those extensions.