Missing character: There is no * in font cmr10

Problem Description

After a recent update of Tex Live, a prompt will appear when compiling files with xelatex:

This problem has not been fixed in subsequent updates of Tex Live in the past few days.

investigation

1. I encountered a similar error before because the fonts were incomplete. Later, I added two font-related collections and it was solved:
   
   So this time I tried several collections that may be related, and also used Tex Live Shell to regenerate fontmaps, and tried again. Retire the ghostscript version.
   During this process, I also found that the Info window information of some collections indicated that the installation was incomplete. I also deleted and reinstalled it, but it had no effect;
2. Searching the Tex Live program directory, I found that the recently updated file (xetex.dll) does contain this string prompt:
   
   Since the official does not intend to solve it, it may be a newly added feature, and we can only continue to find the reason from ourselves;
3. Using the dichotomy method, we quickly located a content that triggered the prompt:
   
   using the text editor's highlight search result function, it is easy to find that the previous line is wrong and the next line is correct.
   They are all caused by false space characters contained in the $formula environment$.
   For example, the hexadecimal content corresponding to the place that should be a space is: f4 80 80 80. Changing it to an English space or deleting it will solve the problem.

in conclusion

False spaces may be copied from other files, and occasionally spread by copying formulas within files.
After copying content from other files, it is best to clean up the spaces first, especially in formula environments, to avoid including false spaces and causing redundant prompts.