Not surprising... if you're going for groundwork, you need to be able to neutralize at least one leg or they'll use them to get out. The straight arm bar might have worked on an opponent that would quickly submit to pain.
Actually, no, she wasn't pinned. Sakuya was using her legs just to hold the arms... the barred arm was between her thighs, with her butt on the ground against Tenshi's side for leverage (with the other arm held in her ankles, so Tenshi can' t use it to try and free herself). There was no actual downward pressure on Tenshi to pin her. Without that and with the legs completely free, the opponent has a lot of options to escape. Well, except in real life, that won't happen in this situation, because you don't need to pin when you have a hold that will get you a very quick submission. But this is fiction, and getting a submission from Tenshi isn't likely to be quick (if it's possible at all), so Sakuya should be focusing on pinning holds.
The Pain of Reversed JointsShe's completely locked into it!It's like it'll snap at any moment with the force she's exerting!Her opponent is a celestial.Ngh!Well...Episode 926: Full Strength Hold Down.From the Artist's Pixiv page.I was stretched out too, if only for a sec.Whoa! She's forcing herself!Will her arm...Nnh... nnggh!She's forcing herself to try and stand!Nnnnggh...That attack's no laughing matter.That's probably saying she's going all out.I'm sure she realizes this means she can't break Tenshi.Will her arm be all right?!