When I use the Keystore I made to make an Android build, I can access in-app-purchases and everything expected from a production build. If anybody else uses the same Keystore to make an Android build, the in-app-purchases don’t work and Facebook complains that the key is wrong.
I figured it out. We’re using Unity’s Asset Server for source control, and it somehow lost the check-in of the Keystore, so when everyone grabbed from the server, they got reverted back to the old one. Happily, I still had the correct Keystore so I could check it back in.