I am running Vegas Pro 20 (build 326). When I place a video clip on the time line and play it, the video plays ok in the Video Preview Window. However, If I check the box to enable External Monitor, the Video Preview freezes, even though it is playing fine in the External Monitor. As soon as I uncheck the External Monitor box, the video in the Preview window resumes playing as normal. The video also freezes whenever I open any other window such as Text Box editor. This is very frustrating as I cannot see the size and placement of the text in relation to the video until I close the Text Editor window.

I still have Vegas 19 Pro (build 636). A similar thing happens with the Text editor, but the Video Preview does continue to play when I enable the External Monitor which causes it to freeze in Vegas Pro 20.

My system is as follows:

Win 10 Pro (22H2 - 19045.2728)
Core i9 - 10980
32GB Ram
nVidia GeForce RTX 3070 (528.49)
2TB SSD

Some have mentioned that it may be an issue with the video driver, but again V19 does not have the same issues as V20