The issue is Ahsoka. I like her, but Vader’s line, “when I left you I was a learner, now I am the Master,” indicates Vader was never a Jedi Master as Anakin.
Or a full knight for that matter. Just another bit of OT continuity Lucas either forgot or ignored when writing the prequels.
I’ve always been of the mindset that we fans can often take dialogue too literally. This is a case of that for me. After 20 years of experience, a medical doctor would certainly acknowledge that they were still a learner in their first few years out of med school, even though they may still hold the same title. The line is just meant to establish the dynamic of the characters in the scene, it’s not meant to be expository.
To make the case for this particular topic, you’d be better off using Obi-Wan’s line to Luke that Vader “was a pupil of mine until he turned to evil.” Because that IS meant to be expository. But for me personally, even that is vague enough to allow for some wiggle room, as many young (and even older) professionals in the real world have pupil/mentor relationships with others in their field who simply have more experience.
I think we often need to give character dialogue the freedom of having been generalized, abbreviated, obfuscated, etc… because that’s just how humans communicate. We use shorthand, we leave out unnecessary details, we avoid painful discussions. And it doesn’t always paint an accurate picture of our experiences. Just my 2 cents.