When I updated my Eclipse from 2020-03 to 2020-06, some garbled characters occurred, so I will summarize the cause and remedy.
Symptoms are the same as the URL below. A part of the entered Japanese comment etc. becomes a? Mark surrounded by □. The character code is not the cause. https://qiita.com/segur/items/60b3711c0c9207d23b2f
Changing the font to one that supports Japanese as described in the URL above will solve the problem, but it will take time to find a good monospaced font in Japanese, and it will be a pain for those who like the default font Consolas.
Upon investigation, this bug has been confirmed since around 2018, but it seems that it was once resolved in 2019-12. https://bugs.eclipse.org/bugs/show_bug.cgi?id=536562 However, it is thought that it has recurred with the 2020-06 update. (As of 2020-06-30, there was already a report on the above URL)
Therefore, I decided to downgrade because I thought that the garbled characters would disappear if I returned to 2020-03. I referred to the URL below. https://stackoverflow.com/questions/28030294/how-to-downgrade-my-eclipse-luna-to-eclipse-kepler
If it was translated into Japanese with Pleiades, Help-> About Eclipse IDE -> Installation details -> Installation history
So, I was able to downgrade by selecting the date before the update (I think the latest one is possible) and selecting "Restore to last saved state" at the bottom. As a result, the phenomenon that a part of the comment etc. becomes a? Mark surrounded by □ no longer occurs. There is a possibility that the bug will be fixed in a future update, but I do not know when it will be fixed, so if you have already updated to 2020-06 and do not want to change the font, I think that it is a good way to try it. ..