How to reproduce:
1. Open the attached "1306311_Repro" project
2. Open the "NewTest" Scene
3. In the Hierarchy select the "NewA" GameObject
4. In the Inspector expand the "Sub Emitters" section found in the Particle System Component
5. Change the current Sub Emitter to "SubB" by dragging the "SubB" (child of the "NewB" GameObject) GameObject onto the Sub Emitter GameObject field
6. When asked to re-parent select "Yes, reparent"

Expected results: The Sub Emitter is re-parented successfully, no errors are thrown in the Console when entering the Play Mode
Actual results: The Sub Emitter is not re-parented, the "Sub-emitters must be children of the system that spawns them" error is thrown in the Play Mode

Reproducible with: 2018.4.31f1, 2019.4.18f1, 2020.1.17f1, 2020.2.0a19
Could not test with: 2020.2.0a20, 2020.2.2f1, 2021.1.0b3, 2021.2.0a2 because the Editor froze when trying to reparent Sub Emitter