Why is a perfect IV Pokemon no longer 'perfect' after evolution/powering up in Pokemon Go?

You likely used the wrong level value for calculating your IVs. Different sites handle this differently, but if you calculate it manually you would know that for each powerup cost, there are 2 possible odd levels (if the Pokemon is fresh, since wild Pokemon only show up at odd levels), and 2 possible even levels.

Either the IV calculator you used assumed it was at a lower level than it actually was, or you had previously powered it up, hence giving you an incorrect value when initially determining it was perfect.

Alternatively, if you powered it up an odd number of times after checking the IVs for the first time, it's possible that you or your IV calculator didn't account for it.

The answer is rounding error. The CP and HP calculations are rounded down to the nearest integer, so when the numbers are small, perfect IV and almost perfect IV can give the same results. However as the numbers get bigger, which happens with powerups or evolutions, the rounding errors become less important and it's possible to distinguish perfect from almost perfect.

The IV levels for that pokémon are Attack=15, Defense=14, Stamina=15. I think about pokémon levels as numbers that go up in increments of 0.5, and in that scheme, dust=1600 corresponds to 4 possible levels 13, 13.5, 14 or 14.5. When the Cloyster CP was 848 it was at level 14.5. However, it must have been powered up a lot, because as a Shellder, its down at level 7 that perfect IV would have been indistinguishable from the almost perfect IVs for your pokémon.