You need to revalidate and save the application rule rather just saving.
Could you use browser debugger tool(F12) and inspect the div structure, verify if it has the dom element created for this property?
I have tried replicating your scenario in Pega 7.3.1(though the reported issue could occur due to other condition) and was getting result as expecting, When rule was able to modify the presentation of property on UI layer.
This leads me to doubt regarding your when rule condition.
For verification, could you follow steps:
1. Navigate, Designer Studio->User Interface->Gallery->UI Elements->Text Input(Landing Page) (Landing Page menu may differ in older version, just navigate to UI Gallery in that case)
2. Open "pxUIGalleryTextInputRead" section rule and modify the last text input field in the section.
3. Configure an OOTB "When" rule, "Never" or "Always" and verify if it appears on the screen or not? Share the result.
Hope it helps you in isolating the root cause of the reported issue at your end. Kindly notify.
I have tried the above and working fine in Designer Studio>UI gallery.i have modified read only when condition to "Always" for Auto complete field and it displayed as read only in Designer Studio>UI gallery.
But read only condition is not working in our application screen.There is no issue with my read only when condition is also.