Hi,
Hard to say, since I can only guess what is the value of LinkedItems[${INDEX}]. But I guess it contains https://127.0.0.1:5001/index.html? Then Get Request works exactly as expected. At that keyword you do not repeat the complete url that you had already provided at Get session. You provide the last part of the url that will be attached to the url from the session.
So I guess, when LinkItem[${INDEX}] contains the value index.html this should work.
Out of curiosity: you use a very old version of SeleniumLibrary. What version of RequestLibrary do you use? You might want to upgrade both.