Designers should be designing/refining the solution at the PBI level whilst it's still on the backlog - they're part of the Product Owner team.

Putting them into the main team to work on design during the same sprint is far too late.

Leaving juniors alone on a team is obviously stupid. The answer is definitely not to go back to silos by discipline, the failure of which led to cross-functional teams in the first place but to actually give juniors support which this is being used as a proxy for. Organising across discipline for that isn’t silly but much better is to have someone to work under that will provide mentorship.

We used to share designers between teams at a company I worked for. It worked nicely, but the approach has its drawbacks as well, in that the person designing an extension to something might miss a lot of context that the previous person did. Especially if you constantly switch people around, you run into this.

Regardless, that doesn't address the plight of the junior. The junior shouldn't be put alone in a team and expected to effectively manage everything in that beach-ball graph, let alone be expected to come up with proper designs by themselves. That's a fault of the person putting the junior there, not a fault in the design of the whole team.

I think the same could be said for any junior on an autonomous agile product team – designer, developer, analyst, etc. If they're junior the PM will end up just giving them work to do and they'll need to just do it.

The solution is having those more senior in the discipline, whatever discipline it might be, close at hand. Either in the team, or providing cross-team review, mentorship, standards, etc.