When setting SharePoint Online document properties from Flow, you will run into issues if the document is locked, i.e., someone has it open. In this case, SharePoint throws HTTP 400 that cannot be caught by the built-in retry-logic of the Update Item Flow action.
Simplest Do Until loop I came up with can be seen below. I didn’t find using Scope action necessary. In case you need to re-use this elsewhere in your Flow, it is quite straight forward to copy the Do Until action and paste it elsewhere. Just remember to add Set Variable action before each Do Until and set the fileLocked variable to true.
At first, the two Set variable actions were a bit confusing, the first one is only set to run after the SharePoint Update Item action has succeeded (and in that you set the fileLocked to false). The second one, however, is set to be run if the previous Set variable action is skipped (and in that you set the fileLocked to true), and as the first one is skipped if the Update Item fails, we then know it did NOT succeed.
It feels a bit weird to have the second Set variable (Set variable 2) as fileLocked variable value is not changing, but this is the high level logic people seem to do this so there may be some room for further improvement.