Created attachment 208870 [details]
Created attachment 208871 [details]
It was displayed correctly in 12.0.
12 and 12.1 were installed newly newly, and tried whether you recurred.
This phenomenon recurred by an upgrade to 12.1 and 12.1.
This problem doesn't form by 12.0R.
If it's in case of ssh, a problem doesn't occur.
I'd suggest gathering as many technical details as possible and opening an issue with the mosh developers: