We recently added a debug mode to the eviction server in WT-4477. That added some randomization about when to choose lookaside eviction behind the WT_CACHE_EVICT_DEBUG_MODE flag. There was a bug in that code which means that we override the random selection immediately:

We need to remove the line here:
https://github.com/wiredtiger/wiredtiger/blob/develop/src/reconcile/rec_write.c#L642

The value for r->las_skew_newest was chosen in the above if/else.