Short description of error
This might fall into the same boat as anything related to T45964, although it does seem to be "new" in 2.8x.
After performing a loop cut and slide operator, you can adjust the properties in the operator panel (albeit with caveats from T45964). However, if you instead open the Adjust Last Operation panel, it will list a different operator name (Loop Cut, not Loop Cut and Slide), and while you can change properties there, those changes have no effect, even though they are reflected by the operator panel.
This didn't use to happen in 2.79b, though IIUC the "Adjust..." panel is different in 2.8x from the "Redo Last" of the 2.7x era.
Exact steps for others to reproduce the error
- Load the default scene.
- Go into edit mode on the cube.
- Add a loop cut, e.g. via the shortcut or using the active tool
- Expand the operator panel in the bottom left and adjust properties. Note how those adjustments do have an effect on the result.
- Go Edit -> Adjust Last Operation
- Note the different operator name in that newly opened panel (Loop Cut vs. Loop Cut and Slide).
- Try adjusting any properties in the newly opened panel. Note how the operator panel in the bottom left picks up these changes, but at the same time they have no effect on the operation.