This is evolution 3.22.0-2 under Debian Linux, with the relevant system information included at the end of this message.

Steps to reproduce:

1. Start Evolution and configure an account.
2. In the E-Mail view, click New to start composing a new message.
3. If not already in HTML mode, switch from Plain Text to HTML mode.
4. Type two lines of text, e.g.,

a
b

5. Move the cursor to the end of the first line (after "a").
6. Delete the contents of the first line either by pressing BACKSPACE or by pressing CTRL-BACKSPACE.

Expected result:

Contents of the first line ("a") are deleted, with the cursor now at the beginning of the (now empty) first line.

Observed result:

Contents of the first line ("a") are deleted, with the cursor jumping to the end of the message (after "b").

Other notes:

The bug is not triggered by DELETE.

It is not triggered if the line still has text on it (after the cursor) either. For example, starting with text

ab
cd

putting the cursor after "a" and pressing BACKSPACE does not cause it to jump to the end, nor does putting it before "c" and pressing BACKSPACE. However, the bug is triggered regardless of whether the line is the first line in the message. For example, starting with

a
b
c

and BACKSPACE-ing "b" does cause the cursor to jump to after "c".

Debian Release: stretch/sid
  APT prefers testing
Architecture: amd64 (x86_64)

Locale: LANG=en_AU.utf8, LC_CTYPE=en_AU.utf8 (charmap=UTF-8)

Versions of packages evolution depends on:
ii  dbus                   1.10.10-1
ii  evolution-common       3.22.0-2
ii  evolution-data-server  3.22.0-2
ii  libc6                  2.24-3
ii  libcamel-1.2-59        3.22.0-2
ii  libclutter-gtk-1.0-0   1.8.2-1
ii  libecal-1.2-19         3.22.0-2
ii  libedataserver-1.2-22  3.22.0-2
ii  libevolution           3.22.0-2
ii  libglib2.0-0           2.50.0-1
ii  libgtk-3-0             3.22.0-1
ii  libical2               2.0.0-0.5+b1
ii  libicu57               57.1-4
ii  libnotify4             0.7.6-2
ii  libsoup2.4-1           2.56.0-1
ii  libwebkit2gtk-4.0-37   2.14.0-1
ii  libxml2                2.9.4+dfsg1-2
ii  psmisc                 22.21-2.1+b1

Versions of packages evolution recommends:
ii  bogofilter         1.2.4+dfsg1-8
ii  evolution-plugins  3.22.0-2
ii  yelp               3.22.0-1

Versions of packages evolution suggests:
pn  evolution-ews                   <none>
ii  evolution-plugins-experimental  3.22.0-2
ii  gnupg                           2.1.15-3
ii  network-manager                 1.4.2-1