Thanks for testing.  The patch has been pushed to master, in commit a76a6bf6.

More economical patch is attached.

(file #55416)

Transforming the recipe to something that one can copy-paste
(and adding line numbers to make the miswrapping more obvious):

echo -n "one two three four five six seven " >tsf
echo -n "eight nine ten eleven twelve makesadozen " >>tsf
echo -n "andthencomesaverylongwordthatis" >>tsf
echo "probablysomethinglikeacommithashorsimilar" >>tsf
src/nano --ignore --softwrap --atblanks --linenumbers tsf

Start this nano in an 80-column terminal (for example a default xterm), then type: 1 2 3.

See that after typing 3 the "andthencomes..." is placed right after "makesadozen" instead of on a third line by itself.

Now type: 4.  The "andthencomes..." is pushed to the next line, where it should have been anyhow.

Now type: <Bsp>.  The "andthencomes..." moves back to where it was, wrongly, right after "makesadozen".

(This worked correctly before version 6.4, commit 0e9bef34.)

To (hopefully) reproduce use an 80 columns wide terminal. Start nano with `nano --atblanks` and generate a text with the following words all on 1 line:

foobar foobar foobar foobar foobar foobar foobar foobar foobar looongerword verylongwordorrathertextorwhateverlikeforinstanceweburlsandsuchstuff

Press M-$ for soft line wrapping. The result should be 2 lines with only the "verylong..." word on the 2nd line. Place the cursor at the start of the 1st line and start typing 12345. Once 5 was inserted observe that "looongerword" was correctly wrapped to the 2nd line but the "verylong..." word should actually have been wrapped to a 3rd line but it is still shown in the 2nd line.

Additionally (but this would be a consequence and not the cause): After typing 5 type Cursor-Down several times and observe that the cursor moves to the 2nd line but does not move to the 3rd line.

An ASCII art "screenshot" of my terminal at this stage would be

+---------1---------2---------3---------4---------5---------6---------7---------8+
|12345foobar foobar foobar foobar foobar foobar foobar foobar foobar             |
|looongerword verylongwordorrathertextorwhateverlikeforinstanceweburlsandsuchstuf|
|f                                                                               |

Helpful for debugging might be the observation that the wrapping continues to work as expected once the  1st line is made 1 char longer (e.g. by inserting a 6 after 5):

+---------1---------2---------3---------4---------5---------6---------7---------8+
|123456foobar foobar foobar foobar foobar foobar foobar foobar foobar            |
|looongerword                                                                    |
|verylongwordorrathertextorwhateverlikeforinstanceweburlsandsuchstuff            |
|                                                                                |

Nano 7.2 on Debian GNU Linux with libncursesw6 6.2+20201114-2+deb11u2 using a VTE w/ TERM=xterm, and I also reproduced this in a tmux window w/ TERM=tmux-256color.