I found the solution!
if ( bbp_get_topic_forum_id() === $forum_id )
This is the branch!
To @casiepa
Before 5.3, we used 5.2.4 WordPress.
bbpress is 2.5.14 is the last version.
If default=21, I was able to input 7 characters in Japanese!
Apparently, 18-20 can be up to 6 characters, 21-23 can be up to 7 characters, 24 can be up to 8 characters in Japanese. This was a new discovery!
When bbPress was changed to 2.6, wordpress was upgraded to 5.3 as well. As you say, it may have an impact. I will try a little more to find out what’s wrong.
For the time being, I decided to set 60 instead of 20.
Thank you for your cooperation!
Thank you for thinking together, @casiepa!
I’m sorry for the space after the dollar. I missed it.
But I fixed it but it didn’t work.
For Ver2.5, if $default = 20, you should have been able to send up to 20 characters in Japanese.
