PutItem replaces
PutItem writes a whole item and overwrites any item with the same key.
Why
PutItem writes an entire item. If the key is new, you get a new item. If an item with that key already exists, PutItem replaces it wholesale: it does not merge.
Add a track, then overwrite track 001 with a version that drops the seconds attribute. Look at the row: the old attribute did not survive. That is the footgun and the feature.
PutItem is write-or-replace by key. To change part of an item without losing the rest, use UpdateItem.
These rows are real and already here. Running an operation against them downloads a DynamoDB-compatible engine, about 850 KB, and executes it in this tab.
A new key, so a new item appears in the Chill playlist.
Run an operation to see the raw engine response.