I was able to reproduce the issue described by Alessandro.
Initial state
After
Field 1 lost focus
<!DOCTYPE html>
<html>
<head runat="server">
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:FormPanel ID="_frm" Title="Form" DefaultAnchor="100%" Width="400" Height="150" runat="server">
<FieldDefaults MsgTarget="Side" />
<Items>
<ext:TextField FieldLabel="Field 1" runat="server" />
</Items>
</ext:FormPanel>
</form>
</body>
</html>
But when
Field 1's AllowBlank is set to false, everything works as expected.
<!DOCTYPE html>
<html>
<head runat="server">
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:FormPanel ID="_frm" Title="Form" DefaultAnchor="100%" Width="400" Height="150" runat="server">
<FieldDefaults MsgTarget="Side" />
<Items>
<ext:TextField FieldLabel="Field 1" AllowBlank="false" runat="server" />
</Items>
</ext:FormPanel>
</form>
</body>
</html>
I am gonna investigate it and keep you posted.