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Niantic is going to fix the Level 40 Bug!!
http://pokemongo.nianticlabs.com/en/post/ver-update-110117
It was only a matter of time! This resolves the lvl 39 vs. lvl 40 issue. =]
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Answers
No, the fact that some players found away around the bug, wasn't an exploit.
As someone with a family with shared devices where we log into and out of accounts I was fairly certain all the evidence was powering up to 40 was fine, and not was the bug.
Everything else you try to do on multiple devices:
Catch same pokemon --- fail
Spin same stop --- fail
Raid same mob --- share balls.
All made me 99% Convinced since powering to 40 worked, it wasn't the bug, the button disappearing was.