He was already like this when he fought Jiri, it's just who he is. Dude thinks he's playing some sort of chess game instead of being in a fight...no sense of urgency at all.
The thing is I kind of understood it logically against Jiri as a strategy - he's just going to try and brawl you and his poor defense just walks him into shots, plus he doesn't check leg kicks well at all. So in that fight Rakic was able to really chew his legs up with powerful kicks and avoid getting into extended pocket exchanges, where if he pushed forward it would've invited the fight Jiri wanted (but ended up getting anyways int he end).
Ank actually does have good technical kickboxing and can do a lot of things Jiri can't - he can check leg kicks pretty well, he lands his own leg kicks pretty well, he can patiently close distance with technical footwork to not just lunge into counters, he can slip and counter in the pocket.
I said it before in other threads - the way to beat Ank for most guys without the most elite of elite leg kicking games (Jan/Poatan) is to put him on the back foot. Ank doesn't want to brawl, he doesn't like when guys are blitzing him with combinations, he wants to patiently stalk you and back you up into the cage and use reads/feints to draw out attacks he can counter while making you tire yourself out from all the movement.
I guess Rakic's tools/tactics/strategy are truly limited, he tried to wrestle/clinch-fight at the end when he was gassed and out of ideas but it's ineffective against a quality wrestler/grappler like Ank so he just ended up stalling out the end of the fight instead of breaking into space and trying to blitz and actually sit down to punch with power.
Maybe Rakic just doesn't have those tools (ability to throw in combination or sit down on his power shots) or the confidence (to push forward against a very tough and layered fighter in Ank), but that was the clear game plan to be implementing. Instead they were running away the entire fight spamming high-kicks and eating straight lefts thinking he was easily up 2 rounds. The first round was a toss-up honestly, he could've lost that one as well and been down 2-0 instead of up 2-0!
He's probably just who he is at this point and isn't going to change/evolve much. This was a fight he didn't deserve even, he was coming off of back-to-back #1 contender fights that he lost against Jan and Jiri, so they give him a third #1 contender fight against Ank and he does the exact same shit of fighting off the back foot spamming kicks. He probably won't ever get a chance to fight for a title now, and this was a good as opportunity as any to steal Ank's juice/spot and just leap-frog himself into contention.