For example the control ignores IsTabStop and TabIndex properties for keyboard tab navigation. It behaves differently on Android, iOS and UWP.

MaskedInput ignores VerticalOptions

sample MaskedInput definition:

_Telephone = new RadMaskedInput
            { 
                Mask = "[(][0-9]{3}[)] [0-9]{3}-[0-9]{4}",
                MaskType = MaskType.Regex,
                WatermarkText = "Phone # (999) 999-9999",
                RejectedSymbolErrorText = "Only Numbers Allowed!",
                InvalidInputErrorText = "Invalid Input",
                WatermarkTextColor = Color.Black,
                ErrorColor = Color.Red,
                DisplayedTextColor = Color.Black,
            };

            _Telephone.InputValue = "2223334444";
            Grid.Children.Add(_Telephone, 0, 0);

and the result:

RegEx mask doesn't work at all.

A very basic test mask, single digit required: 

<telerikInput:RadMaskedInput x:Name="maskedInput" Mask="^[0-9]{1}$" MaskType="Regex">

Result:

Anything can be typed in.

In the ApplyMaskFinished event, e.IsAccepted == true.

On top of that, the control has problems when selecting, overtyping and inserting content.

Type something:

Select all text:

Type "1":

I'd expect the entire existing content to be deleted and new value to be "1".

Same goes for inserting a character:

Attempt to insert "4" between "3" and "5":

"5" is overtyped. Inserting doesn't work.

Press the delete key to make the input "1.11.2020.":

Or press the backspace key: 

The ApplyMaskStarted event isn't fired.

It's not about the mask, since it's set as follows (it's not working at all, as reported in the other bug):

Mask="^[0-9]$"

The control has problems when selecting, overtyping and inserting content.

Type something:

Select all text:

Type "1":

I'd expect the entire existing content to be deleted and new value to be "1".

Same goes for inserting a character:

Attempt to insert "4" between "3" and "5":

"5" is overtyped. Inserting doesn't work.

System.ArgumentOutOfRangeException: Count cannot be less than zero.
Parameter name: count
   at System.String.Remove(Int32 startIndex, Int32 count)
   at Telerik.XamarinForms.Input.MaskedInput.MaskParser.ConstructCandidateTextForMaskTypeRegex(String oldText, Int32 startIndex, Int32 selectionLength, String enteredText, Boolean userDeletes, Int32& insertIndex)
   at Telerik.XamarinForms.InputRenderer.UWP.MaskedInputFormsTextBox.CreateCandidateText(KeyRoutedEventArgs e, Int32& actualInsertIndex)

The issue can be reproduced in SDK Browser application MaskedInput/Features/Regex Mask example IP and email masks when enter symbol like ., @, etc.