I found the cause In Vegas Pro 19, when I press control z to run the wrong job while working on subtitles, the frame size suddenly changes and the preview doesn't work even if I change the size.Do you too?
You should really be a beta tester for Vegas : ) You are a Korean user, right?
This is a test of legacy text which isn't maintained (meaning it's not fixed anymore). Do you see the same problem with either Titles and Text or ProType Titler? Those are more likely to be fixed.
I just updated to the newest build (361). I tried both ProType and Titles and Text and when I press undo it momentarily shows 0 x 0 pixels but then resets to the correct number. It seems to be working correctly here.
I tried again with 361. With ProType I can replicate your issue- I picked Center or Center Baseline, dragged to the timeline and then changed text. When I hit undo it said 8 x 8 pixels.
I haven't been able to replicate this error with Titles and Text yet.
Thank you for your answer. My English is a little poor, so I just need to understand that my friend had the same problem as me, right? It's not just about me, right?