Undo operation in code block is not as expected

Logseq version: 0.9.19
I create a code block by typing ```c```
and then insert a b c to code block, like this

a
b
c

image

Then type ctrl + z to undo my operation. what i expect is only word ‘c’ is undo :

a
b

However, below is the result after i press ctrl + z :

image