There are 2 factors to consider when the values entered in the Memo field do not populate in the Description field of the Invoice when billing Billable Time for customers.
1. Confirm if the preference for Copy Time Memos to Invoices found by navigating into Setup > Accounting > Accounting Preferences > Time & Expenses subtab > Time Tracking portlet is enabled.
Check this preference to have invoices include memos entered on billable time and unset to leave the Description field empty.
2. Verify if the checkbox for Combine Time Items on Invoices is enabled by navigating into Setup > Accounting > Invoicing Preferences.
When this is turned on, only the first Time Tracking will show its related Memo and the rest will have empty Description fields.
Only one Memo is copied over from the Time Tracking since it will only show one line item (having the preference set to combine) when printed out.