Investigate: Proposer of previous generator not joining the next

[jakmeier]

I observed the following in logs:

• Participant 1 was proposer for a signature, broadcasts START but then times out before the signature finishes.
• Later on, participant 3 becomes the proposer for another attempt with a new set of participants.
• Participant 1 is now deliberator and ACCEPTs the posit from participant 3.
• Participant 3 broadcast START and particpant 0 and 2 (both deliberators) process it within 30ms of each other. Nothing happens on participant 1.

Full scenario: #681 (comment)

Task: Try to replicate the scenario with a test. If this can be replicated, debug and fix the issue.