The given value for a taxonomy field was not formatted in the required <int>;#<label>|<guid> format

[VincentVH]

Full errormessage:

The given value for a taxonomy field was not formatted in the required <int>;#<label>|<guid> format.
clientRequestId: 8ded8119-c957-40be-a079-e928d4c4badf
serviceRequestId: 8ded8119-c957-40be-a079-e928d4c4badf

I have a created a flow that generally receives a response from a REST API, processes the response into variables and then feeds 10+ variables (json formatted) to a HTTP POST to update the properties of a SharePoint document set. After a few days of trial-and-error, the flow runs perfectly now in various circumstances.

However, if I re-run the flow on a document set that I have processed before (the flow is triggered manually 'for a given file on SharePoint'), the first action 'get File Properties' fails with the above error message.

This is a screenshot of a succesful HTTP POST, with the relevant value highlighted in blue:

As you can see: the value is formatted correctly. Also, the HTTP POST is sucessful, and the browser shows the exact value from the termstore (at least the correct labels). For some reason, a subsequent 'get File properties' results in an error about the syntax of this value.

Some more information:

1. The field that is being updated is a multi-value managed metadata field (although the above example only shows the field being update with a single value). I can confirm the same behavior on document sets where the field is updated with multiple values

2. If the field is updated with a multi-value string, this is the format (taken from another example):

"4;#Class 03|<TermGUID>;#1;#Class 05|<TermGUID>;#2;#Class 16|<TermGUID>"

3. To make this work, I am updating a hidden field, called by its 'internal Name', as described online in many blogs and articles, also here: https://www.aerieconsulting.com/blog/update-using-rest-to-update-a-multi-value-taxonomy-field-in-sha...

4. If I run the flow a first time, then update the managed metadata field manually and run the flow a second time, the error is gone.

Conclusion: The syntax I'm using, is valid for the HTTP POST command to the REST API, but it is not acceptable for the 'get File Properties' action.

What am I doing wrong?