Loma is certainly not invincible, but judging things by the first few rounds would by remiss if it went unmentioned that Lomachenko stated straight after the fight that he used the first two rounds as a feeling out process. No coincidence that round 3 comes and he starts lighting him up.
I don't think the Salido fight was jitters, it was a mix of pride, inexperience and some misguided sense of honour. He refused to hit low back, he refused to complain to the ref, so he tried clinching which as far as I could see was the only way he'd left himself to deal with the problem of the low blows.
Then this fight comes and we see him not only theatrically complaning about pretty much every single low blow but he intentionally lands a heavy one himself. The result? His opponent gets deducted two points for low blows and probably wasn't far off a DQ if he dared land one more. I don't know if his team had much say in the ref for the contest but it wouldn't surprise me as he was pretty hot on the low blows which seemed marginal in many cases.
All that said Walters is a hard fight for him and Rigondeaux, well to me that's the fight of the decade not Pac/May. And I'd favour Rigondeaux if they meet sooner rather than later as he has the edge in speed, is less conventional and moves so well. If it happens post-2 years I think the pendulum swings toward Loma who will only get better with experience and age will be even more in his favour.