This |mach try auto| push had an xpcshell task that ran too many manifests:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=cb6453d697573582d3dc30fdc08451d25c9c7439&selectedTaskRun=dj4-aqH7Q2633gnd2l4GBA.0

It may be trying to place too many manifests into a single chunk. XPCShell is tricky as they run in parallel, so they need to use a separate chunk duration:
https://searchfox.org/mozilla-central/source/taskcluster/taskgraph/transforms/tests.py#295

Though the linked task is Android, do Android xpcshell tests run in parallel? Looks like it's excluded from that DYNAMIC_CHUNK_MULTIPLIER dict.

(In reply to Andrew Halberstadt [:ahal] from comment #0)

Though the linked task is Android, do Android xpcshell tests run in parallel?

Android xpcshell tests do not run in parallel.

https://searchfox.org/mozilla-central/rev/227f22acef5c4865503bde9f835452bf38332c8e/testing/xpcshell/remotexpcshelltests.py#580

Maybe they need a multiplier that increases the total chunks instead. Though I'd like to try and reproduce the issue first.