Flake: If a device list update goes missing, the server resyncs on the next one

[clokep]

https://github.com/matrix-org/synapse/actions/runs/5699129687/job/15447723403

[clokep]

Maybe a regression from #1359? (Or the Synapse change: matrix-org/synapse#15975)

[anoadragon453]

https://github.com/matrix-org/synapse/actions/runs/5951648487/job/16142138945#step:8:9

[clokep]

https://github.com/matrix-org/synapse/actions/runs/5953174350/job/16147214483?pr=16171

[DMRobertson]

https://github.com/matrix-org/synapse/actions/runs/6009539638/job/16299183502

[sumnerevans]

https://github.com/matrix-org/synapse/actions/runs/6085861208/job/16511081329?pr=15503

[anoadragon453]

https://github.com/matrix-org/synapse/actions/runs/6095919477/job/16540512047?pr=16260

[DMRobertson]

https://github.com/matrix-org/synapse/actions/runs/6218153002/job/16874260219?pr=16338