How to reproduce:
1. Open the attached project (SerializeReferenceUndoBug.zip)
2. In the Hierarchy window select the BugExample GameObject
3. In the Inspector window under the 'Behavior Example' Component press the button 'Null'
4. Undo the last action (CTRL + Z or Edit -> Undo Inspector)

Expected result: variables 'Value' and 'Integers' get set to their previous values ('Value' = 1, 'Integers' = {1,2,3})
Actual result: variable 'Value' gets set to its previous value while 'Integers' do not

Reproducible with: 2019.3.15f1, 2020.1.0b10, 2020.2.0a12
Could not test with: 2018.4.23f1 (SerializeReference not introduced)

Notes:
1. The 'Create' button sets the SerializedProperty managedReferenceValue to the ExampleClass (which has the 'Value' and 'Integers' variables). Undoing this works as expected - the values get properly reset
2. The 'Null' button sets the SerializedProperty managedReferenceValue to null