Reproduction steps:
1. Open the attached project "case1266303.zip"
2. In the Project window click on element named "Test asset"
3. In the Inspector window right-click on "Element 0" and select "Delete Array Element"
4. Undo the action
5. Observe the "Element 0" Integer field in the Inspector window

Expected result: The Integer field value is the same as before deleting
Actual result: The Integer field value is 0

Reproducible with: 2019.4.6f1, 2020.1.0f1, 2020.2.0a19
Could not test with: 2018.4.25f1 (namespace name 'SerializeReferenceAttribute' could not be found)

Note: When deleting array element Editor throws this error: Could not reach source property name 'entries.Array.data[0]' while extracting diffs, the reference does not exist in the source serialized data