It started early in the IDW run, where everything was rebooted and everyone was getting a new origin story. Writer Simon Furman decided to address Arcee's role as the token lady Transformer by explaining that Cybertronians are, by default, genderless, but a mad scientist turned Arcee into a female-gendered Cybertronian and caused her to freak out and become homicidal.
It was, as the kids say, problematic.
Subsequently, Furman left the series and a new group of writers were brought on for a soft relaunch they called "Phase Two". The main three writers were John Barber, James Roberts, and Mairghread Scott. Scott wrote a pretty good blog post about the Furman story, why it was offensive, and how she and the other two writers intended to address it.
All three writers introduced a number of new female Transformers (explaining that the reason we'd never seen them before is that none of them were from Cybertron, they were all from other planets that Cybertronians had colonized millions of years earlier, unbeknownst to the current generations of Cybertron).
Barber took Arcee's origin and massaged it into something less jaw-droppingly misguided. Arcee is trans; it was her choice to transition; and while she was subsequently tortured by a mad scientist, that's not the reason she identifies as female.
Barber still depicted her as something of a violent loner, to start with -- the Snake-Eyes of the group -- but one of the major character arcs over the course of his run was having her open up. She meets Aileron, a Transformer from the planet Caminus, where the Primes are literally worshipped as gods. Aileron is religious and naive; over the course of the series she and Arcee each sort of become a little more like the other. Aileron learns a few painful lessons about the world -- such as, Optimus Prime isn't a god, he's just a guy -- while Arcee learns to open up, trust people, and become part of the community rather than an outsider. And then eventually

Anyway there was a big continuity reset after that and the Transformers/MLP crossover isn't canon anyway (it opens by gently making fun of fans trying to figure out where crossovers are supposed to fit into continuity), but between Arcee flirting with Rarity in Transformers/MLP and having a female partner in Transformers Galaxies (not Aileron this time, somebody new named Greenlight), it would appear that Arcee liking the ladies is one piece of continuity that has survived the latest reboot.