It is possible to set an input text box to both “Required” and “Read-Only” either through the Editor or directly in Velo code. One might do this if the read-only input text box error condition is related to some other event happening on the page.
The validity object for the input text box shows “valuemissing” as true, but the input text box never changes to the viisible error state, even when calling updateValidityIndication.
Perhaps this is working as designed, but it seems to me that if you can set the required attribute you should be able to see the error state if the read-only input text box has no value.