Seems this is a bug and can be reproduced.
1.) Open Patch my PC and see which program's can be updated. I normally have like around 6 to 7.
2.) I like to update some, like Chrome and Thunderbird via the application itself. So while PMPC is open, I open Chrome, update it, then Thunderbird same way and maybe another program.
3.) After that is done I use the "rescan" option in PMPC and the updated programs are not shown as outdated anymore. Now I click to select the remaining applications which should be updated.
4.) After doing that, the update butten is blanked out, I can only click to uninstall the selected applications. This is odd because some version I always worked like that and there was no issue.
5.) When you close PMPC and then open it again and then click to select the outdated apps, then I can select to update the selected apps.
Conclusion, something is preventing to update apps one select after doing a rescan where some apps are updated externally.