Blank form inserted upon reassignment of Master Page

[Former Community Member]

Master Page #1 = A4; MP#2=A4; MP#3=Custom (4"x5").

I insert Body Page #2 which looks like MP#1.

I change BP#2 reference to MP#3 ...

- The program changes BP#2 to look like MP#3

- Then the program inserts BP#3 by itself which also looks like MP#3 but is overlaid with an A4 sheet.

I try to insert a checkbox onto BP#2

- The program inserts BP#4 and inserts the checkbox on that page instead ...



I think the program is trying to take over - how do I fight back?