I had renamed some pokemon to their IVs and stat profiles a while back (via the command; /ivs <slot>), and now it's 0, regardless of what number it used to be. Unfortunately, this is not a visual bug and the stat did reduce.

I tested to see if every pokemon in my pc had followed suit and I found some inconsistencies. Upon further investigation, I noticed a heavy correlation between ball type and defense IV. Those who were captured in a pokeball have an IV of 0. Those who were captured in any other ball actually have a value higher than 0. The interesting bit is that I never caught a pokemon using a pokeball, and only the most RECENT pokemon I caught were of different ball variant. I checked across the 650 pokemon in my PC. Thus, older pokemon caught turned into pokeballs and had their defense IV altered somehow.

Defense IV change not lining up:
https://imgur.com/Ih1SvgO

Ball type vs Defense IV:
https://imgur.com/rhoX7Z7

Updating from what version exactly? That bit of information is important.

From 9.1.3 to 9.1.5