A couple things. First is to see what's in the hosts.txt file. Second thing is to move the Export-Csv command outside of the ForEach-Object statement. If you leave it how it is, it will only return the last object it processed in the csv file. I've posted an example of how it should be.
Get-Content C:\Hosts.txt | ForEach-Object {Get-ADComputer $_ -Properties Name,LastLogonDate | Select-Object Name,LastLogonDate} | Export-Csv C:\Output.csv
I'm not able to replicate your problem and the only thing different is the contents of the hosts.txt file so it would be helpful to see it.