I use format/write quite a bit, but never exactly this way... ;-)

write effectively keeps a running tally of the number of lines printed on the current page. (To be more precise, $- holds the number of lines remaining on the current page.) Apparently, $- is not getting reset when you close RF and then open it again. You should put

$- = 0;

right after close RF;. And in order to get that to work, you need to select your filehandle so that $- is associated with RF.

Here's a sample:

use strict;
use warnings;

format RF =
testing
.

format RF_TOP =
top_of_form
.

for ( 1 .. 3 ) {
    open (RF, ">testfile$_") or die "Can't open file testfile$_: $!";
    select(RF);

    write RF;
    write RF;
    close RF;
    $- = 0;
}
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