Dynamic dropdown change value, child dropdown list still old value

Hi experts,

I create two BCO(city, town), then define two element use it.

city

A

B

C

town

A1     A

A2     A

A3     A

B1     B

B2     B

C1     C

C2     C

C3     C

C4     C

I set codelist context mapping,

I chose A, then A1,A2,A3 only see, I chose town is A2.

Then I changed the city B, but the town is still A2. (direct click save is ok)

I think it is not the correct control.

Q1、It can be set to null or B1(the first data)

Q2、Also, can I write absl(event-afterModify) to control custom field properties(ex. Visible, Mandatory, Readonly...)

Regards,

Daniel