flexibility of user phrase and it's mapping in chewing

[hsufit]

I just watched the issue discussion in #113, and here comes a problem :
It seems not really support this feature in chewing

I add four different user phrase
a) 是大將軍（ㄕˋ ㄉㄚˋ ㄐ一ㄤ ㄐㄩㄣ）
b) ㄕㄉㄐㄐ（ㄕˋ ㄉㄚˋ ㄐ一ㄤ ㄐㄩㄣ）
c). ㄆㄞㄆㄞ（ㄆㄞ ㄆㄞ ㄆㄞ ㄆㄞ）
d). (:з」∠)（ㄅ ㄅ ㄅ ㄅ ㄅ ㄅ ㄅ ㄅ）

When I type bopomofo of (c), it shows (c).
If I change a word in it, there is no way to go back to choose the mapping into ㄆㄞㄆㄞ,
and so does in (d).

When I type bopomofo of (a), it shows (a) and (b)
Then, I change it into 是大江軍 and then back to 是大將軍
It can still find mapping of 是大將軍 and ㄕㄉㄐㄐ

Is this a bug of libchewing, or there is still some restriction need to be add on user phrase?