If our texts attributed to Justin Martyr were composed around the 140s-150s as is often said, then Justin's ignorance of Acts must raise questions.
However, earlier, the Epistula Apostolorum 30-31 is aware of Acts.
For Justin, the twelve disciples just upped and went out to all the world preaching as the resurrected Jesus had commanded them on Easter Sunday. No Pentecost, no replacement for a lost disciple (not even a missing disciple), no Jerusalem conflicts or persecutions, no Paul (not even a Peter, iirc), no Hellenists or Stevens or Philips, and no matching Simon Magus tale. All twelve just went out to all the world after the resurrected Jesus appeared to them -- contrary to Matthew, Mark, Luke, John and Acts.
The twelve going all over the world right after the alleged resurrection to make converts is also in the interpolated ending of gMark and Aristides' apology.
Even after Acts is named and quoted by Irenaeus, the same Irenaeus still kept the twelve early travels all over the world to make converts,
including among Gentiles (as for Justin Martyr) in his 'Demonstration apostolic':
"His disciples, the witnesses of all His good deeds, and of His teachings and His sufferings and death and resurrection, and of His ascension into heaven after His bodily resurrection----these were the apostles, who after (receiving) the power of the Holy Spirit were sent forth by Him into all the world, and wrought the calling of the Gentiles"
Still later, Origen wrote in in 'Commentary of the gospel according to Matthew' X, 18:
"And the Apostles on this account left Israel and did that which had been enjoined on them by the Saviour, "Make disciples of all the nations," and, "Ye shall be My witnesses both in Jerusalem and in all Judæa and Samaria, and unto the uttermost part of the earth." For they did that which had been commanded them in Judæa and Jerusalem; but, since a prophet has no honour in his own country, when the Jews did not receive the Word, they went away to the Gentiles."
Irenaeus & Origen, even when they knew about Acts, kept repeating the rosy goody idealistic false facts (as compared to what shows in Acts) of the twelve going into the whole world to make converts.
So I do not see any problem about Acts being available to the author of Mk 16:9-20, Aristides & Justin and still, these writers keeping the idealistic picture of Jesus' own disciples making Christians all over after the alleged resurrection.
But if the idea of this idealistic spread of Christianity was prevalent in the 2nd century and beyond, why does Acts say the opposite if written in the same time period (the disciples stay in Palestine, and the churches outside Israel are created by non-Jesus disciples, including the very important one in Antioch, and then by Paul & Barnabas, and then Paul only)?
So that one reason why Acts was written earlier, and also why Acts was not mentioned up to Irenaeus' times: because it went against the idealistic scenario for the early world-wide spread of Christianity: by Jesus' disciples themselves.
Cordially, Bernard