Steps to repro:

1. Download attached project: https://drive.google.com/open?id=1JfrNt_hJ3M8FqsfJUkuurdSOB0XWaw0X ;
2. Open 't1' scene;
3. In the Hierarchy select 'Reflection Probe' gameobject;
4. In the Inspector under the Reflection Probe component text click on a crossy icon to open Probe Origin reflection Probe editing mode;
5. With Editor gizmoz grab in the Scene view grab the reflection probe origin and drag it to the edge of the reflection probe box by Z axis;
6. Notice that reflection probe's box offset position is (0;0;-5)
7. Undo by either pressing Ctrl/cmd+Z or by selecting Edit>Undo Modified Reflection Probe Origin.

Expected result:
Box Offset position is restored to the default value (0;0;0),

Actual result:
Box Offset's position on modified on Z-axis got inverted after undoing the modified probe origin position (0;0:5). See attached video.

Notes:
- Reproducible in 2018.3.0b1, 2018.2.7f1, 2019.1.9f2, 2017.4.11f1;
- Reproducible on Windows 10 and OSX 10.13.6;
- Reproducible only when manually dragging the probe origin; not reproducible while changing box offset position by inputting value in the Box Offset field.