When rebooting after installing the latest PureOS (Amber) Update on my Librem 15v4 through Software I got the warning that ./grub/grub.cfg could be compromised and also had to set a new default boot system (of the many options that didn’t tell me anything I chose the first). So, far I haven’t noticed anything not working.
Anything to worry about? Please excuse my ignorance.
Potentially any operating system update changes one or more files that are monitored for changes.
So if you get a warning about changed files …
… if you have just applied an update then no stress
… if you have not just applied an update then look more closely.
It’s a very good security practice that you pay attention to the warnings you received. 
With that said, the advice irvinewade posted is worth following, namely that your update may have changed the grub files (to include a new kernel perhaps), and with the new files a new file checksum may have been created. Checksums are used to detect tampering but may also detect legitimate changes to files as well since legitimate changes will affect the checksums.
It’s wise to double check things as you’re doing.