SOPHOS FIREWALL WEB FILTER OLD LOGS IN LOG VIEWER

You see all the details in logviewer when you search for the up.

Beginning to sound like your reporting system is corrupt.
ian

There are limitations on the live log that can cause this issue : 

https://doc.sophos.com/nsg/sophos-firewall/19.5/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Logs/index.html#local-logs

Basically they are the first thing to get thrown away if the log partition gets slightly full, I come across "disappearing logs" a lot.

Normally you can fall back to the other report formats available on the firewall itself, but as you've spotted there isn't a Blocked Report (for a reason I can't fathom) similar to the Custom User Report (which only reports on successful web access).

It may be too late now, but if you had your XG paired to a Central Dashboard you would have 7 days worth of log history stored there you can use to run an ad hoc report against (which is free).

You could also consider getting an Advanced Reporting license for Central that boosts the amount of data retention and gives more flexible reporting options.

Regards