Lately it seems many are rejecting Saul/Paul as a legitimate apostle of Lord Jesus. I guess I'm trying to figure out if this is a new concept or a fairly old one.
A speculative answer:
It's a natural attack angle for theological liberals and others who reject the authority or veracity of the NT to take, as rejecting Paul's apostleship allows you to reject 50% of the NT out of hand, as well as the clearest expositions of most standard Christian doctrines.
As attempts to sever Christianity from its historical orthodoxy escalate, so will attacks on Paul.
I have interacted with a few 'organizations' and individuals who strongly oppose Paul's teaching and apostleship. Below are a few links that may be of interest. In my estimation the centerpiece of their hatred of Pauline doctrine is grace. Just as Paul was accused of promoting licentiousness in his day:
God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged. But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man) God forbid: for then how shall God judge the world? For if the truth of God hath more abounded through my lie unto his glory; why yet am I also judged as a sinner? And not rather, (as we be slanderously reported, and as some affirm that we say,) Let us do evil, that good may come? whose damnation is just. - Romans 3:4-8
and was accused of despising the Law:
And when the seven days were almost ended, the Jews which were of Asia, when they saw him in the temple, stirred up all the people, and laid hands on him, Crying out, Men of Israel, help: This is the man, that teacheth all men every where against the people, and the law, and this place: and further brought Greeks also into the temple, and hath polluted this holy place. - Acts 21:27-28
so are there today those who would take occasion to misrepresent the grace of God in Christ and attempt to bring the unstable and unlearned back under the ministry of condemnation
There is nothing new under the sun and the enemy of our souls has no need to use any but the tried and true methods of spiritual murder. Here are the links I mentioned above. There are certainly more, some more aggressively against Paul than others. All of them massively mishandle (perhaps deliberately) Paul's words and doctrine. Many of them present as meek and gentle. Please be careful and, if you are not yet apprehended of Christ, stay away from such as these.
It's gone on since Galatians
A major theme of Galatians is a defence of Paul's apostleship as genuine and his gospel as genuine. It's not subtle; Galatians 1 and 2 arguably have Paul's defence of his apostleship as their main topic.
As Galatians is most likely the very first NT book written, or close to, this is not new.
It's an important theme in 1 and 2 Corinthians as well. (Thanks NigelJ for suggesting I add them).
Paul doesn't have a resounding endorsement of his own apostleship in 1 Corinthians 15:8
And last of all He appeared to me also, as to one of untimely birth.
or "abnormally born" (NIV).
@Dottard Biblical hermeneutics translates ..
The noun here is ἔκτρωμα = untimely birth = miscarriage or lifeless abortion, or birth beyond term.
Thats not a very positive self-assessment of his credentials.
Personally, I think this is humility, however for his opponents (which there are many) a statement of weakness is another yard stick to beat him with. The opponents will simply say apostleship is the 12/11 disciples selected by Jesus in his earthly ministry, but obviously I question their underlying motives.
Here I'm stating that a key part of being an apostle is one who has seen the resurrected Christ, this being the basis Jesus has selected Paul.
To address the allegation of eisegesis (comments), Paul goes on to say,
9 For I am the least of the apostles and do not even deserve to be called an apostle, because I persecuted the church of God.
I struggle the inaccuracy of my exegesis.