It's a really trivial bug, if you try to write something like :3: the number 3 and the : disapear.
if you write :0: nothing happen
if you write :a: nothing happen
if you write :(1-9): the whole group disapear
if you write :10: nothing happen
if you write :11: or :12: the whole group disapear.
if you write :13: nothing happen
if you write :2 : and then erase the space like :2: nothing happen.
Well, I tried to explan it as well as I can, sorry if I have made some spelling mistakes, I'm not very good at English.
Actually not disappearing, rather autocorrect is applying emoji and the font you are using does not have a valid glyph for the Unicode value substituted.
Tools -> Autocorrect -> Autocorrect Options: "Replace" tab
You can change your font (Symbola, or Segoe UI Symbol)
You can edit the Autocorrect emoji blocks and delete the :n: values
You can edit the Autocorrect emoji blocks and change the delimiter.
Anyhow--duplicate of bug 97191, closing.
*** This bug has been marked as a duplicate of bug 97191 ***
Sorry for duplicate, I've looking for information before posting a bug report but I couldn't find anything.