The dialog "reboot required" shows at the start of the installation process, and clicking the "Reboot" button on it immediately reboots the system without installing the updates.
Thank you for the bug report. Unfortunately we were not able to get to it yet. If your system is still running Plasma 5, can we ask you to please check if this is still an issue with Plasma 5.27.12 (the current LTS version)? The PK Updates Widget has not been ported to Plasma 6, so if your system has updated to that, this report isn't relevant anymore. If you can still reproduce the bug, please change the Status to "CONFIRMED". Thanks.
🐛🧹 ⚠️ This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information, then set the bug status to REPORTED. If there is no change for at least 30 days, it will be automatically closed as RESOLVED WORKSFORME. For more information about our bug triaging procedures, please read https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging. Thank you for helping us make KDE software even better for everyone!
🐛🧹 This bug has been in NEEDSINFO status with no change for at least 30 days. Closing as RESOLVED WORKSFORME.