There may be several possibilities for receiving a warning regarding peak luminance of 10,000 Nits. Film scans may have dirt or dust that can hit 100 percent white, but typically, such errors are caught during the color session. A more probable reason that is often the cause can be the introduction of graphic elements, such as title cards or subtitles.
Graphics and Transitions
Traditionally speaking, the main titles tend to be introduced during the color correction process and Colorists will color correct them appropriately within the project and thusly these titles will have no problems . The problem tends to arise when foreign language subtitles are supplied post color correction, and the luminance values are not adjusted for the HDR grade.
What happens in this scenario pertaining to subtitles is that the luminance value(s) that most post houses and third party sub-title vendors are used to and work with on an every day basis, will not work for Dolby Vision. The reason for this is that they tend to work in a peak luminance space of 100Nits in Rec709. When these elements are pulled in and applied as, say, a Cinecanvas XML or other automated sub-title generator, the luminance values of these corresponding metadata files MUST BE altered to be viewed correctly in High Dynamic Range. Any such elements need to be fixed before Dolby Vision masters can be rendered out.
In your grading session, lower the luminance of titles/cards/flash whites into a reasonable level any time you found them in 10k nits by default. Re-analyze the shots if necessary.