Emulator

Game Name?

All Wii Titles

Game ID? (right click the game in the game list, Properties, Info tab)

Yep

MD5 Hash? (right click the game in the game list, Properties, Verify tab, Verify Integrity button)

all of them

What's the problem? Describe what went wrong.

If you start up Wii Netplay beyond 5.0-13923, it ignores NAND building procedure and will load completely different saves on each computer.

What steps will reproduce the problem?

You don't even need a partner.

Start up a Netplay Host Window
Turn off all forms of save loading and syncing
Make sure the Wii game you're having has a savefile. I used Super Mario 64 because it was easy and supported GameCube controllers.
Observe that even though no save should have loaded, it loaded a save anyway.
Go to older build before this one and realize that it works correctly.

Is the issue present in the latest development version? For future reference, please also write down the version number of the latest development version.

Yes

Is the issue present in the latest stable version?

No

If the issue isn't present in the latest stable version, which is the first broken version? (You can find the first broken version by bisecting. Windows users can use the tool https://forums.dolphin-emu.org/Thread-green-notice-development-thread-unofficial-dolphin-bisection-tool-for-finding-broken-builds and anyone who is building Dolphin on their own can use git bisect.)

5.0-13923 -> https://dolphin-emu.org/download/dev/da534c7d757f90e6801847636777e8147e63b10f/

What are your PC specifications? (CPU, GPU, Operating System, more)

Intel Core i7-6700K, GTX 1070, Windows 10

Is there anything else that can help developers narrow down the issue? (e.g. logs, screenshots,
configuration files, savefiles, savestates)

That pretty much sums it up I guess. Took a lot of convincing from different people to convince me that Wii Netplay was actually broken but they were right.