Hi,
I am having this problem also. I have a flow that I am using to add a concatenated value to a Single Line of Text column in SP. The flow is supposed to pickup each entry in my list, concatenate a standardised prefix with the SP ID for each particular entry.
The outputs all show that the field is populated, but in SP it is still blank for every entry. The field is "Asset Number".

My expression for Concatenate is concat('CAMS-ASSET-',outputs('Get_item')?['body/ID']), I have a get items followed by Get item and Update item both inside a "apply to each".

Just not sure why it's showing up as successful output in PowerAutomate but still blank in SP.