I just played through Beyond Oasis for the first time, working through my backlog while stuck at home, and I reached a point in the final dungeon where it seems like it's pretty easy to put the game in an unbeatable state. Here's what happened to me.
I reached a point where there are 3 ice crystals blocking a path. I figured I should go back a few rooms and summon the fire spirit to destroy them, which he did easily. I then proceeded past, fought some enemies, and then came to a row of spikes that was too long to jump over and moved too fast to run through. I couldn't figure out a way of getting past them, so I ended up just brute forcing it and taking a bit of damage. However, once on the other side I reached a gap where I clearly needed the shadow spirit in order to cross.
This is where things get tricky. I tried going back through the castle and finding a spot where I could summon the shadow spirit. There was a crystal earlier in the dungeon where I was able to do that, and then I tried to make my way back to the gap I'd just been blocked by. However, on the way back you need to fall into a pit in order to reach the area with the gap, but having the shadow spirit currently summoned makes it impossible to fall into pits, as he (usually helpfully) catches you.
This means that you would *have* to summon the shadow spirit at some point *after* falling into the pit, but there were no crystals from which to summon it after that point. I resorted to looking on Gamefaqs, and a guide said to make sure to not break all 3 of the ice crystals blocking the path (the ones I had the fire spirit destroy), as you'll need at least one of them left to summon the shadow spirit.
Luckily I had an item on me that would let me summon the shadow spirit from anywhere, but those are one-time-user and I only found 1 in the entire game, and inventory space is pretty limited. So if I hadn't found that item, or had already used it before this point, I'm pretty sure I'd have needed to start the game from scratch and lost those 6 hours.
Also, to be clear, since you don't have direct control over the spirits, when I brought the fire spirit into the area with the ice crystals, he just goes off on his own and destroys them, so you'd have to be pretty careful to dismiss him after he destroys the first 2 in order to avoid this situation. Seems like a pretty mean potential softlock to put 20 minutes before the end of a game.