mode -a 5 m -0 bug???

[rolter09]

5e9516c893a0cf6ac00afe2ea89e84d7:bug@this

password
bugathis
buga

table
working
a=@this
a=5

not working
a=@this
b=5

not working
a=@
b=g

working
a=@
a=k

working
a=a
b=A
a=4
a=@
b=/\
b=æ

not working
b=a
b=A
b=4
a=@
b=/\
b=æ

is this a bug???