When collection has dot in name, source struct's collection field only contains first part of collection name.
For example: collection name is test.collection, collection field will contain only test part:

Struct

{version=0.10.0.Final,connector=mongodb,name=connector,ts_ms=1571228697000,db=testDb,rs=myReplica,collection=test,ord=1,h=3870166902378819437}

In 0.9.+ versions, collection field wasn't present in source struct.