Mask extension on Textbox - bug

I'm submitting a...

Bug report (I searched for similar issues and did not find one)

Current behavior

When the mask of a TextBox is 99/99/9999 I can systematically makes it fail displaying something wrong and not accepting figures (or wrongly accepting them).

Expected behavior

I should be able to type any patterns that respects 99/99/9999

Minimal reproduction of the problem with instructions

Here is a repro. If you tap on Add then Remove then Add then Remove then Add, you'll see that the date doesn't display properly. And if you try to edit it, it behaves strangely.

Environment

Nuget Package(s): Microsoft.Toolkit.Uwp.UI

Package Version(s): 5.1.1

Windows 10 Build Number:
1809

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [x] October 2018 Update (17763)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Mobile
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
2019 16.0.3