How do I use a nonstandard file descriptor for reading a file into an array with mapfile?

mapfile -t -u 7 arr < textfile gives me bash: mapfile: 7: invalid file descriptor: Bad file descriptor Other, more verbose, methods of reading files line-by-line do allow for such descriptor, e.g.

read_w_while() {
while IFS="" read -u 7 -r l || [[ -n "${l}" ]]; do
  echo "${l}"
done 7< textfile

The standard descriptor, 0, is used quite a lot. Does using such descriptor make scripting more secure from interference? My experience is that so far I only witnessed, I am a Ubuntu Desktop user, such interference when using while IFS="" read -u 7 ... with the descriptor 7. What might be the reasons of such interference.