DMS not replicating addition of new columns to source

this is expected behaviour

mongodb as source for dms can be configured two ways

document mode and table mode

in document mode whole row irrespective is migrated as single column along with _id

> db.myCollection.find()
{ "_id" : ObjectId("5a94815f40bd44d1b02bdfe0"), "a" : 1, "b" : 2, "c" : 3 }
{ "_id" : ObjectId("5a94815f40bd44d1b02bdfe1"), "a" : 4, "b" : 5, "c" : 6 }

to

oid_id	                        _doc
5a94815f40bd44d1b02bdfe0	{ "a" : 1, "b" : 2, "c" : 3 }
5a94815f40bd44d1b02bdfe1	{ "a" : 4, "b" : 5, "c" : 6 }

thus during replication it will bring forward all columns as one doc

while in table mode (seems to be your case)

target columns are mapped as

oid_id	                        a	b	c
5a94815f40bd44d1b02bdfe0	1	2	3
5a94815f40bd44d1b02bdfe1	4	5	6

as mentioned at https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.MongoDB.html#:~:text=To%20create%20the%20target%20columns the target columns are limited during full load and are as per the docs being investigated (default 1000) for setting target columns thus any further additions of columns during replication wont be replicated

Say the source following record is added in replication

{ "_id" : ObjectId("4a94815f405d45d1b02bdf51"), "a" : 3, "d" : 5, "c" : 7 }

target would have

oid_id	                        a	b	c
4a94815f405d45d1b02bdf51	3		7

i hope the explanation is clear