gdbtui source window does't display Unicode text

In Ubuntu 20.04, I discovered that the gdb 9 TUI source code window doesn't display properly the UTF-8 characters (Chinese, Cyrillic, etc.) but displays correctly only the ASCII ones. For example, the following source code

printf("Здравей, свят!\n");

appears in the source code window as

printf("^^P^ ^P^ ^Q^┗↑@^^P^ ^P^ ^P^ ^P^ ^^Q^┗↑╝^^P^ ^Q^ ^Q^ \n");

The command windows is not affected (when I type non-ASCII characters into it, they appear OK).

Additionally, I found out the following:

(1) This bug was introduced in version 8.3;
(2) It affects only some CPU architectures.

In more detail, I tested version "Debian 8.3.1-1" on two 64-bit machines with the same version of Debian (bullseye/sid), one with PowerPC, and the other with SPARC. The bug shows up only on the SPARC machine, not the PowerPC one. I then built the generic version 8.3.1 on a 64-bit ARM machine. The bug didn't show up there. And then, I built it on a 64-bit MIPS (big endian) machine, where the bug did show up. Repeated this with version 8.3. The bug showed up again. When I built the generic version 8.2.1 on the same MIPS machine, the bug didn't show up. A "diff -u gdb-8.2.1/gdb/tui gdb-8.3/gdb/tui" shows a lot of changes made in the TUI code of version 8.3 to add some colourisation to the TUI, and the bug was probably introduced during this process. Sorry that I can't help more to isolate the bug. But it's quite strange that it manifests itself only on x86-64 (as I had found out previously using Ubuntu 20-04), SPARC and MIPS, but not on ARM and PowerPC. I suppose that because of this, it will be rather hard to pinpoint.

This bug is similar to the already fixed bug 1275210 where I also described it but then realised that it has a different origin and is therefore a different bug, so I report it as a new one now. Someone already reported this bug as bug 25296 in the gdb Bugzilla which I confirmed there.