_reindex API issue

When I try to _reindex an existing index to new index, I see drop in document volume. I’m trying to do it as the original is created with wrong mapping.

_count API on original results as below

{
"count": 3273105,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
}
}

_count API on new results as below
{
"count": 267199,
"_shards": {
"total": 2,
"successful": 2,
"failed": 0
}
}

I see a huge difference. The process didn’t throw any error as well. Any pointers!

Providing the reindex request you made would be helpful.

Hi Mark,

Below is the exact request made (using sense plugin)

POST /_reindex
{
"source": {
"index": "IdxOld"
},
"dest": {
"index": "IdxNew"
}
}

I passed the parameter wait_for_complete=true to request.
Few other information:
Source Index has 5 shards/1 Replica in settings and went with default mapping (all analyzed strings).
Target has a valid mapping, which it is intended for. It has 2 shards/1 Replica in settings.
No errors were thrown

From here I can't tell what happened those kinds of reindex requests work
fine for me.