REST API - Get Commits on BRANCH Since date - Some commits are ignored

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Hi:
I’m using GIT LAB community edition installed on our servers.
I need to mimic the following command-line command: git log TP_6.0.7…HEAD.

TP_6.0.7 is a TAG present in branch TP_6.0

Only way I’ve found to try to get same resulta was:

  1. get the creation date of TAG TP_6.0.7 (2019-05-10)
  2. request all commits present on branch TP_6.0 since the date: the creation date of TAG TP_6.0.7

Problem is that several commits are missed.
I’ve set the date to 2019-01-01 the I’ve got the missing commits, we work using Merge Requests and I’ve found 2 commits that have same commit messages with dates > 2019-05-10 on fields: “created_at”,“authored_date”,“committed_date”

I do not know if info I’m providing is enough, but I’m very confused.
GITLAB VERSION CE 11.6.2