From code:
Option Explicit
Sub LockThisSheet
	LockSheet("Sheet1")
End Sub
Function LockSheet(passedSheetname) 
	Dim oDoc 		As Object : oDoc	= ThisComponent
	Dim oSheet 		As Object : oSheet	= oDoc.Sheets.getByName(passedSheetname)
	Dim pPassword 	As String
	If IsMissing(pPassword) Then pPassword = ""
	oSheet.UnProtect(pPassword)
	oSheet.Protect(pPassword)
End Function
Why does the icon not appear in front of sheet name ?

On the other hand, it will show after moving such sheet.

[ Fedora 32 Workstation Cinnamon Desktop Spin ] [ LibreOffice 6.2.8.2-2 ]