we quite clearly were told that
1. luffy is a prodigy when it comes to haki and fighting. and he actually focuses on training his haki and spends time on it.
2. haki blooms in extreme difficulty fights.
3. luffy after training with an haki specialist - learnt the basic forms of all 3. he only learnt the advanced versions through battle.
robin meanwhile after training with sabo for two years - had picked up nothing related to haki. if you're telling me that she mastered armament hardening in the 3 weeks between zou and wano - while infiltrating orochi's banquet and not training her haki.
i think that's just bad writing.
1. Luffy is a prodigy, so he respectively achieves massive heights. In just over two years he goes from not knowing about Haki to surpassing Hyogoro, fellow conquerors Chinjao and Doflamingo within the same arc, and fellow prodigious conqueror Katakuri with his own specialty. With Robin we're talking about a pirate with twenty years of experience training with high level fighters in just the basic application of ONE color.
2. Haki probably blooms while you're getting your ass beat in the biggest fight of your life, right? We have the scans now so we can see that's exactly what happened. Usopp taps CoO without trying, god forbid Robin has an ougi after all this time.
3. Robin has a high IQ. We would have no idea how long she really had to figure out CoA but we do know it would be much more time than Luffy used to figure out Gear Second, Third, Future sight, or advanced CoC. "Mastered" is a stretch because she didn't pull it out until it was do or die. You might as well complain about Zoro's Haki in Alabasta tbh.
Btw, we're never told that "full body" is any sort of advanced application. It's pretty much just a last ditch effort. As soon as Luffy pulled out Gear 4, a similar concept, Law mentioned how he was using too much. Similar to how Maria straight up comments on Robin trying to do the same thing. Maybe you don't like the writing and that's fair, but it's not bad due to inconsistency.