If I do this ...
martin@MartinsAspire ~]$ mapfile arr
a
b
c
[martin@MartinsAspire ~]$ echo "${arr[@]}"
... I see ...
a
 b
 c

[martin@MartinsAspire ~]$
... explaining an error in an arcane shell-library I a developing.

Another thing: As part of debugging this I tried delimiting with null instead of newline - but that flops because in command-substitution (when I am e.g. building a data-set to input as a here-document) null-chars are dropped (at least with a warning about it).