Should this be reported as a bug:

The $interface variable in /ppp profile on-up|on-down scripts has values like this:
*f00001, *f00002, *f00003 ... *f00019, *f0001a, *f0001b (Small Letters)

If an interface with one of the interface ids from above is added in an interface list and I print the members of the list I get this:
* My focus is on ppp interfaces that are already down, that is why the members are displayed with their interface ids, not with their names.

Here the interface names are with Capital Letters and above are with small letters. This is causing me problems in some scripts. It would be better if the names are written in the exact same way everywhere in ROS. Should this be reported as a bug?

it's not a bug
$interface : will gives youe id of interface not the name of interface
if you want get interface name?, use this below

The code you offered me I am already using it.

The $interface variable (when used in ppp-up or ppp-down script) returns the interface id starting with small letter -> *f00001
But when I print an interface lists members then the interface ids are shown starting with capital letter -> *F00001

To me this is inconsistent naming convention inside ROS and is causing me problems in some scripts.

It turns out that the difference in the naming (small, capital letters) is known thing. In order for ppp on-down script to work well the support offered me this idea:
toid $interface

I tested it and works great. I would prefer not to explain it because I have only an assumption how it works. For me this solved all my problems.