(I'm using KMail 17.04.3, which seems to be not listed in the "Version" selection). When opening an email with many recipients (just by clicking on it, not in an own window), the "View complete To list" button is missing. When I click on "View → Headers → Kmail 5.2" (which is also the only option to choose), it appears.
Thank you for reporting this issue in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version? If you can reproduce the issue, please change the status to "REPORTED" when replying. Thank you!
Well, I guess this has been fixed in the meantime – at least I didn't notice this recently ;-)