This has been happening since Onigiri's last maintenance. Pretty self-explanatory. When my Youkai's stamina runs out, my partner, often being Sasaki Kojiro, will ALSO be put away, even when she's still out and running around. When this happens, I can't Vanguard Swap, and she'll only attack sometimes. This makes it VERY difficult to run dungeons when I use her against the boss, as her Vanguard Swap resets when I have to bring her back out.

You sent a bug report about this via the ticket system, right?

Yep. Now it's just a matter of seeing if they'll actually bother fixing it or not.

I've sent tickets for around five different bugs in the past 3-4 months, and they always get fixed after several weeks. New bugs do, at least.