What was missing in Apollos' knowledge when Priscilla and Aquila explained to him the way of God more accurately?

[TonyChanYT]

Acts 18:

24 Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures.

I.e., the OT.

25 He had been instructed in the way of the Lord. And being fervent in spirit, he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John. 26He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him aside and explained to him the way of God more accurately.

Luke seems to be using "the way of the Lord", "the things concerning Jesus", and "the way of God", more or less interchangeably.

27And when he wished to cross to Achaia, the brothers encouraged him and wrote to the disciples to welcome him. When he arrived, he greatly helped those who through grace had believed, 28for he powerfully refuted the Jews in public, showing by the Scriptures that the Christ was Jesus.

The story continued in Acts 19:

1 And it happened that while Apollos was at Corinth,

Apollos left Ephesus for Corinth. Ephesus was in Asia Minor (Turkey). Corinth was located in Achaia (Greece). They were separated by the Aegean Sea.

Paul passed through the inland country and came to Ephesus.

Paul came to Ephesus after Apollos was gone.

There he found some disciples. 2And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.”

They were ignorant of the Holy Spirit.

3 And he said, “Into what then were you baptized?” They said, “Into John’s baptism.” 4And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” 5On hearing this, they were baptized inb the name of the Lord Jesus. 6And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying. 7There were about twelve men in all.

It seems that quite a few disciples in Ephesus, like Apollos, were not up to date on the knowledge of the Holy Spirit.

What was missing in Apollos' knowledge of Jesus?

Jamieson-Fausset-Brown:

This man was instructed in the way of the Lord … knowing only the baptism of John—He was instructed, probably, by some disciple of the Baptist, in the whole circle of John's teaching concerning Jesus, but no more: he had yet to learn the new light which the outpouring of the Spirit at Pentecost had thrown upon the Redeemer's death and resurrection;

Ellicott:

We ask in what the teaching, which is thus described as accurate, was yet defective. The position of Apollos at this stage was, it would seem, that of one who knew the facts of our Lord’s life, and death, and resurrection, and had learnt, comparing these with Messianic prophecies, to accept Him as the Christ. But his teacher had been one who had not gone beyond the standpoint of the followers of the Baptist, who accepted Jesus as the Christ during His ministry on earth. The Christ was for him the head of a glorified Judaism, retaining all its distinctive features. He had not as yet learnt that “circumcision was nothing” (1Corinthians 7:19; Galatians 5:6), and that the Temple and all its ordinances were “decaying and waxing old, and ready to vanish away” (Hebrews 8:13).