Upon submission of blank time card from the Time Sheet Portal, an error notification on the header should show:
Cannot submit a blank time card
If we use the Submit button on the top right, to submit all time cards, the message displays correctly.
If we submit a single blank time card, using the three dots menu on the right of the line, the message will only appear after reloading the page.
Time Sheet Portal is based on service portal. The widget controlling this behavior is "Time Card Grid" on sp_widget table.
If "Time Card Grid" widget has been customized this issue may occur.
Reverting the above mentioned widget to the Out-Of-Box version should get the issue fixed.