bash: IFS "stuck" after temporarily changing it for array building

I've encountered a strange problem after temporarily changing IFS for the purpose of array building:

$ echo "1 2 3" |while read myVar1 myVar2; do echo "myVar1: ${myVar1}"; echo "myVar2: ${myVar2}"; done
myVar1: 1
myVar2: 2 3
$ IFS=':' myPaths=( ${PATH} )  # this works: I have /home/morgwai/bin on ${myPaths[0]} , /usr/local/sbin on ${myPaths[1]} and so on
$ echo "1 2 3" |while read myVar1 myVar2; do echo "myVar1: ${myVar1}"; echo "myVar2: ${myVar2}"; done
myVar1: 1 2 3
myVar2: 
$ echo $IFS

$ echo "1:2:3" |while read myVar1 myVar2; do echo "myVar1: ${myVar1}"; echo "myVar2: ${myVar2}"; done ;
myVar1: 1
myVar2: 2:3

Normally when I change IFS temporarily for any other command than array building (for example IFS=',' echo whatever) its value is changed only during the execution of that, however here it seems as if IFS got permanently changed to a colon (although echo $IFS doesn't show this, which is even more strange...).

Is this a bug or somehow an expected behavior that I don't understand?
I'm using bash version 4.4.18 if it matters...

Note: I know that I can build the same array using IFS=':' read -a myPaths <<< ${PATH} and then IFS gets reverted to the default value normally, but that's not the point: I'm trying to understand what actually happens in the example I gave above.

Thanks!

Solution

You're just setting variables, not setting a variable followed by executing a command (ie, the way you build array is a pure variable assignment, not a command, hence both assignments become permanent).

The issue with an IFS of : not showing up in echo $IFS is caused by shell parameter expansion and word splitting.

Consider:

$ IFS=:
$ echo $IFS

$ echo "$IFS"
:

When a parameter expansion is not quoted, it undergoes word splitting afterwords. From the manual:

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

and

The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators. If IFS is unset, or its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and <newline> at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words.

So when IFS is a colon, splitting a word consisting of just a colon results in a (single) empty word. Always quote your variables to prevent unexpected gotchas like this.