Unable to flag / trigger "Merge when pipeline succeeds" via Gitlab Api (v3/v4) - powershell

Question

So as a part of some tests to automatically accept / merge successful pipelines in our git repository i was running some tests to flag the "merge when pipeline succeeds" feature when the pipeline is still running:
So this button is available when the pipeline is still running and will convert to a green 'Accept merge' button when the pipeline succeeds:
(note that this picture was taken afterwards not to confuse the use-case)
additionally i have set these general settings:
So when checking the Gitlab API Documentation it says i should use the following endpoint:
PUT /projects/:id/merge_requests/:merge_request_iid/merge
when using the parameter ?merge_when_pipleline_succeeds=true it should flag the button.
However when i call this endpoint when the pipeline is still running (i built in a wait for 10 mins while testing this) i get the following result:
i am getting a Method Not Allowed. My assumption is that the endpoint i am using is correct because otherwise i would've gotten a bad request / not found return code.
when checking the gitlab merge request i am seeing that indeed the flag is not set to true:
However, when i manually click the blue button the mergerequest looks like this:
Also if i let the pipeline finish and then proceed to call the merge api (w/ or w/o the merge when pipeline succeeds flag) it will accept the merge. It just does not work when the pipeline is running (which is odd because even the button itself only shows when the pipeline is running)
so i am wondering what I am doing wrong here.
I am using a Powershell module to call the GitLab API. The Accept part of the module is not official and was made by myself because i found this feature missing.
I am using the same credentials for the API w/ a personal access token to authenticate to the API. Other functionality of the API work with this token like creating merge requests, retrieving status of a current MR and accepting a MR when the pipeline is finished.
I have tried the following variants :
Use the V3 api with merge_when_build_succeeds=true --> nets the same
result
Uncheck the "Only allow merge request to be merged if the
pipeline succeeds" --> nets the same result
Use ID of the merge request instead of IID
use /merge_when_pipeline_succeeds instead of ?merge_when_pipeline_succeeds=true
use True instead of true --> nets the same result

Answer 1

I get a similar issue with the python-gitlab library on v4. It works sometimes when I use:
mr.merge(merge_when_pipeline_succeeds=True)
Where mr is a ProjectMergeRequest object. However, if the MR has a merge conflict in it I get that 405 Method Not Allowed error back.
My best guess is to see if I can apply logic before calling mr.merge() to check for problems. Will update this if that works.
UPDATE: Looks like there is no feature to check for conflicts as of today. https://gitlab.com/gitlab-org/gitlab-ce/issues/41762
UPDATE 2: You can check merge_status when looking at the MR information, so either that attribute or an exception then mr.merge() fails would let you identify when it won't work.