How to reproduce:
1. Create a new project
2. Import the UI Builder from the Package Manager
3. Open the UI Builder window (Window -> UI -> UI Builder)
4. In the UI Builder Hierarchy add two TextField Elements
5. Select the first TextField and in the UI Builder Inspector window mark the Readonly attribute as true
6. Select the second TextField
7. Select the first TextField

Expected result: the Readonly field is set to true
Actual result: the Readonly field is set to false

Reproducible with: 0.5.7-prevew (2019.3.14f1), 0.11.0-preview (2019.3.14f1, 2020.1.0b9, 2020.2.0a11)

Notes:
The same issue occurs:
1. when saving the UXML file after marking the boolean attribute as true
2. with other Elements that have the boolean attributes (Float, Long, Integer)
3. with other boolean attributes (Password, Multiline)