SERVER-41016, we enabled functionality in WiredTiger to represent changes to documents as deltas in cache and in the WiredTiger log (aka journal). That change was primarily aimed at replicated workloads, so the cache pressure more closely matches the size of operations in the oplog.
However, this is the first time we are using size-changing deltas in MongoDB, which is also the first operation that is not idempotent in WiredTiger's log. There have been several test failures that could be explained by such operations being applied by WiredTiger recovery on top of data that already includes the change.
Disable the new modify functionality for collections that rely on the WiredTiger log for recovery. This should not matter for the original cases
SERVER-41016 was intended to address.