Snowplow's GTM SS Amplitude Tag has logic to transform timestamp fields from the ISO8601 format to the epoch format expected by Amplitude.
To take advantage of this functionality, it's important to ensure the field names match the expected format. For example, use x-sp-dvce_created_tstamp rather than dvce_created_tstamp.
The Snowplow Client adds non-common field names with the x-sp- prefix, and these are required for the tag to identify the timestamp field. If the expected fields are missing, it will use the timestamp Amplitude generates when it receives the event, which is likely to be out of order.
We recommend using either x-sp-dvce_created_tstamp or x-sp-derived_tstamp.