[DialogPane] ContentText sometimes not displayed

contentText is not displayed in some cases.
To reproduce, use the test app attached.
Initially, the sample dialog has:
- a graphic
- a content
- an expandableContent
- 2 buttons

Test case to reproduce:
- click on Change contextText: New text
- Show dialog: only the content is displayed, as expected
- close the dialog
- Change content: Reset
- Show dialog: Bug - the contentText is not displayed as it should.