The ISRG Root X1 is from 2015 and is in the Check Point provided Root CA package for a while. I guess this package was not updated a while on your management server. This process was semi-automatic until the versions mentioned in sk173629 and even the automatic part of this semi-automatic process was broken on every environment I saw yet. I mean there was no "update available" banner like shown in sk64521.
I suggest you search for the most recent zip file on your management server. On R80.40, it is here:
/opt/CPshrd-R80.40/database/downloads/TRUSTED_CA/2.0/2.8/updateFile.zip
Download this zip file to your windows machine running SmartDashboard and upload it using Actions -> Update certificate list.
You will get a preview window showing you which CA will be deleted and which will be added, so you can double check before installing it.
If you do not see 2.8 version of this package on your management server, I think you need a TAC case checking why. But I guess you will find it there.