Acts 2:21 is similar to the first part of Joel 2:32 from the KJV:
And it shall come to pass,
that whosoever shall call on the name of the Lord shall be saved.
Acts 2:21
And it shall come to pass,
that whosoever shall call on the name of the LORD shall be delivered: for in mount Zion and in Jerusalem shall be deliverance, as the LORD hath said, and in the remnant whom the LORD shall call.
Joel 2:23, KJV
The Hebrew of Joel has the proper name but the Greek of Acts has a title. This inconsistency becomes more pronounced with Romans (Lord) vs Philemon (Jesus) vs Isaiah (YHWH):
For it is written,
As I live, saith the Lord, every knee shall bow to me, and every tongue shall confess to God.
Romans 14:11
That at the name of Jesus every knee should bow, of
things in heaven, and
things in earth, and
things under the earth;
Philemon 2:10
I have sworn by myself, the word is gone out of my mouth
in righteousness, and shall not return, That unto me every knee shall bow, every tongue shall swear.
Isaiah 45:23