Triage new Byron ThreadNet failure

Karl witness https://hydra.iohk.io/build/7851335/nixlog/4 on PR input-output-hk/ouroboros-network#3380, which is surprising because the PR's contents seems totally unrelated.

I'm therefore assuming it's a flaky failure, but this Issue is to at least confirm that. We should be able to repro it on the `master` branch using the QC seed, eg.

` Use --quickcheck-replay=377968 to reproduce.` seen with his PR's commit `43efa3c0172d5f4d156fb3faaf8cefcfaf49bfa6`

```
simple convergence: FAIL (3.32s)
*** Failed! Falsified (after 8 tests):
TestSetup {setupEBBs = ProduceEBBs, setupK = SecurityParam 6, setupTestConfig = TestConfig {initSeed = Seed 1157156786498218422, nodeTopology = NodeTopology (fromList [(CoreNodeId 0,fromList []),(CoreNodeId 1,fromList [CoreNodeId 0]),(CoreNodeId 2,fromList [CoreNodeId 0,CoreNodeId 1]),(CoreNodeId 3,fromList [CoreNodeId 0,CoreNodeId 1,CoreNodeId 2]),(CoreNodeId 4,fromList [CoreNodeId 0,CoreNodeId 1,CoreNodeId 2,CoreNodeId 3])]), numCoreNodes = NumCoreNodes 5, numSlots = NumSlots 6}, setupNodeJoinPlan = NodeJoinPlan (fromList [(CoreNodeId 0,SlotNo 0),(CoreNodeId 1,SlotNo 2),(CoreNodeId 2,SlotNo 2),(CoreNodeId 3,SlotNo 3),(CoreNodeId 4,SlotNo 5)]), setupNodeRestarts = NodeRestarts (fromList [(SlotNo 5,fromList [(CoreNodeId 0,NodeRestart)])]), setupSlotLength = SlotLength 7.381s, setupVersion = (NodeToNodeV_1,ByronNodeToNodeVersion1)}
params: PBftParams {pbftSecurityParam = SecurityParam 6, pbftNumNodes = NumCoreNodes 5, pbftSignatureThreshold = PBftSignatureThreshold {getPBftSignatureThreshold = 0.3667666666666667}}
Ref.PBFT result: Outcomes [Nominal,Absent,Wasted,Wasted,Absent,Nominal]
delegation certificates: [(CoreId (CoreNodeId 0),[]),(CoreId (CoreNodeId 1),[]),(CoreId (CoreNodeId 2),[]),(CoreId (CoreNodeId 3),[]),(CoreId (CoreNodeId 4),[])]
[(CoreId (CoreNodeId 0),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True}),(CoreId (CoreNodeId 1),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True}),(CoreId (CoreNodeId 2),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True}),(CoreId (CoreNodeId 3),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True}),(CoreId (CoreNodeId 4),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True})]
(CoreId (CoreNodeId 0),SoftwareVersionUpdateLabel {svuObserved = False, svuRequired = Just True})
```