When I updated the Eclipse I was using from 2020-03 to 2020-06, some garbled characters occurred, so I will summarize the cause and the solution.
The symptoms are the same as the following URL. A part of the Japanese comments that I entered becomes a question mark surrounded by □. Character code is not the cause. https://qiita.com/segur/items/60b3711c0c9207d23b2f
As stated in the URL above, changing the font to a Japanese one will solve the problem, but it will take time to find a good monospaced Japanese font, and it will be a pain for those who like the default font Consolas.
When I checked it, this bug was confirmed around 2018, but it seems that it was once solved in 2019-12. https://bugs.eclipse.org/bugs/show_bug.cgi?id=536562 However, it is thought that it has recurred with the update of 2020-06. (As of 2020-06-30, there was already a report at the above URL)
Therefore, I thought that if I returned to 2020-03, there would be no garbled characters, so I decided to downgrade. I referred to the following URL. https://stackoverflow.com/questions/28030294/how-to-downgrade-my-eclipse-luna-to-eclipse-kepler
If you used Japanese in Pleiades, Help -> About Eclipse IDE
So, I was able to downgrade by selecting the date before the update (I think that the latest is as good as possible) and selecting “Restore to the last saved state” at the bottom. As a result, the phenomenon that a part of the comment, etc. becomes a question mark surrounded by □ no longer occurs. Bugs may be resolved in future updates, but I do not know when they will be resolved, so I think that it is a good way to try if you have already updated to 2020-06 and do not want to change the font. ..