Is it normal that hidden part of grouped objects will take part in boolean (whether union or difference)?
Yes, they will also move, scale or respond to any other transformation applied to the visible objects of the same group. Note, you should see a “ghost” of the hidden objects in the group when you try to transform the visible ones. Not the case with a Boolean operation though.
This is why it’s somewhat risky to hide only some members of a group. Note also that locked objects or those on locked layers that are part of the group will also move/transform with the group despite the fact that they are supposed to be “locked”. This has been discussed before, but it was decided that the preserving the integrity of the group should override the individual members hidden/locked settings.
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