Thank you @knelli2. Adding the ExecutePhaseChange action to the worldtube singleton has solved the problem.

There's a subtlety you should be aware of. So the reason the error happened is likely because

1. Elements check for phase change by halting the algorithm and saying they may want to go into a different phase. This > leaves the elements in a valid state
2. The singleton, however, doesn't know this so it just requests the elements for data and waits, leaving it in an invalid >state for switching phases
3. When the algorithm is restarted on the singleton, we get an error because we didn't exit in a valid state.

Putting the ExecutePhaseChange action into the list makes the singleton be in a valid state when switching phases. The subtlety comes in because the trigger for the phase change is checked on both the elements and the singleton which have different time steps/slabs and such. Now the singleton happens to have the same slab size as the elements and is also doing lock-step with the WT boundary, so triggers based on slab number or time value will work for both. But if the slab sizes become different (or you don't do lock-step anymore so the times are out of sync) then you may get a deadlock/error because the elements would try to phase change, but the singleton wouldn't just like currently

Putting this here as record from a conversation with @guilara.