Dear Mario,
Thank you for your answer. Yes, I already tried what you suggest: I tested it with a single folder containing TextEdit files. As a result, the content of all files in that folder ceased to be searchable through Finder and Spotlight; you only can find them by their file name. More generally, the present situation concerning all rtf files is as follows: Finder and Spotlight can search the content of all old files; but as soon as you open them, the content search ceases to work and you have to find them by name. The same happens immediately with all new files. So, trying to rebuild the Spotlight index appears to have the same effect as opening the already existing files, and makes things worse.
It seems that a lot of people are having the same issue under Big Sur. Did someone succeed in fixing it?
All the best,
Max