Yeah, Jesus was never rigid. Oh wait...
“He who is not with Me is against Me, and he who does not gather with Me scatters abroad.” Matthew 12:30
Luke 19:50
But Jesus said to him, “Do not hinder him; for he who is not against you is for you.”
To Jesus, the world is binary - either you are with Him or you are not. There is no middle ground. So it does not matter which way you say it. Either, “if you are not for me you are against me”, or, “if you are not against me you are for me”. To Jesus, they both mean the same thing.
Context is everything. A man was casting out demons. There was no “offense” against Jesus in this verse but the Apostles thought their rights and privileges were being usurped by the man and He corrected them. Matt 12:30 is speaking to offenses against Jesus.
Those are two different scenarios so no reconciliation is needed. Luke 19:50 is talking about somebody doing things in Jesus’ name and Matthew 12:30 is for that who actively oppose or reject Jesus Himself.
In Luke He was talking about the disciples being upset that there were others outside their group who were calling on His name.
Entirely different cases
I’ve always been challenged when attempting to reconcile that scripture with this one:
Luke 19:50
But Jesus said to him, “Do not hinder him; for he who is not against you is for you.”
What version are you quoting from. NIV, KJV, NLT, others stop at verse 48 in Luke 19.