The problem of the resurrected but not yet ascended Jesus telling Mary not to touch him but encouraging Thomas to do so in John 20 is hardly the only problem with John's death and resurrection narrative about Jesus.
John never even gives us the promised ascension at all, despite all the talk in that gospel of the descending and ascending Son of Man.
The absence is not unique to John, however, which tells us that the thinking about all this was, if not fluid, at least not fully formed at the time.
Luke does not reconcile the ascension stories he himself tells in Luke 24:51 and Acts 1:9 with the words of Christ from the cross which he alone records, which imply that Jesus simply expected at death to go to heaven immediately, not to rise from the dead and ascend later, let alone descend into hell in the interim.
Compare Luke's Lazarus, who dies and goes to the bosom of Abraham, while the rich man who ignored him dies and goes to hell (Luke 16:22ff.). This is what is supposed to happen, right? There is no resurrection until "the last day", as Martha informs us (John 11:24). Everybody knows that! But then John's Jesus raises her brother anyway.
And like Matthew's I am with you alway, even unto the end of the world (28:20), the resurrected Jesus in John 21 never really exits the world either. He can appear at any time and say Follow me. Even to one untimely born (I Corinthians 15:8).
Matthew's Jesus doesn't leave in an ascension. He is always present.
For where two or three are gathered together in my name, there am I in the midst of them.
-- Matthew 18:20
The ending supplied to Mark 16, however, agrees with Luke that Jesus ascended to heaven and sat on the right hand of God. Its fascination with signs done by those who believe echos the early Christian history recounted by Luke in Acts, and doubtlessly comes from that part of the tradition and is not originally Marcan.
And [the other malefactor] said unto Jesus, Lord, remember me when thou comest into thy kingdom. And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.
-- Luke 23:42f.
Then came the soldiers, and brake the legs of the first, and of the other which was crucified with him. But when they came to Jesus, and saw that he was dead already, they brake not his legs:
-- John 19:32f.
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.
-- John 20:17
