1. Open attached project's scene "Reflection bug"
2. Select each reflection probe individually and see that "Reflection Probe 1" and "Reflection Probe 2" have each different cubemaps (respectively Reflection 1 and Reflection 2).
3. Select both "Reflection Probe 1" and "Reflection Probe 2" at the same time, and see that they now have the same cubemap assigned.

Note that this doesn't happens when the inspector is set to debug mode, so I assume it's related to the probe's custom inspector.