Found: 5.3.0
Ext.NET Forums' thread: Behavior or LabeAlign="top"

When a field (textfield, combobox, and so on) meets the conditions below:

1. VBox layout with stretch enabled
2. The FieldLabel text set
3. The IndicatorIcon set (i.e. enable the FieldIndicator functionality)
4. Configured with LabelAlign="Top"

This will cause the input field/dropdown to be sized taking the field width into account, which is wrong when the field's label is aligned at top, thus the field is stretched deducing the width of the label, showing as if it were not adjusted to fit the page.

Using the FormLayout for the immediate contained containing the field avoids the issue.

The field indicator feature is specific to Ext.NET so that's most likely not coming from Ext JS unless further investigation proves otherwise.