In Exodus 17 we meet the Israelites camped at Rephidim after leaving the wilderness of sin
Exodus 17:1 NASB
17 Then all the congregation of the sons of Israel journeyed by [a]stages from the wilderness of Sin, according to the [b]command of the Lord, and camped at Rephidim, and there was no water for the people to drink.
In Exodus 18 we are told that Jethro the father in law of Moses visited him while he was already camped at the mount of God
Exodus 18:5 NASB
5 Then Jethro, Moses’ father-in-law, came with his sons and his wife to Moses [e]in the wilderness where he was camped, at the mount of God.
But it is only in the opening verses of Exodus 19 where we are told that Moses and the Israelites left Rephidim and came to the mount of God on the first day of the third month.
Exodus 19:1-2 NASB
In the third month after the sons of Israel had gone out of the land of Egypt, [a]on that very day they came into the wilderness of Sinai. 2 When they set out from Rephidim, they came to the wilderness of Sinai and camped in the wilderness; and there Israel camped in front of the mountain
Is there a chronological issue?.