@Rachmat04 and Siska have logged into bp.miraheze.org, and their contributions disappeared.
Rachmat04 was a while ago, and I noticed it then, but wanted to try it with another user before raising the issue.
Today I stepped Siska through the process, monitoring her contributions list at each step.
After login to meta.miraheze, her bp.org contributions were still visible at https://bp.miraheze.org/wiki/Istimewa:Kontribusi_pengguna/Siska, but her username was still greyed out.
After she visited bp.miraheze.org whilst logged in, CentralAuth created her Siska account on bp.miraheze , and https://bp.miraheze.org/wiki/Istimewa:Kontribusi_pengguna/Siska became empty with her username now a redlink.
If the fix is not easy, it would be helpful for Rachmat04's contributions to be manually fixed, so his account is 'OK', as he will be doing some of the UAT for the migration project.