The issue seems to be that acoustid_id is metadata only found in the file, it is not metadata Picard loads from MB.org normally. That means scripting has no affect on this.
The scripting is applied for data loaded from MB.org, that is the data on the right pane. The acoustid is only inside the file. There is some special handling in Picard, that handles that the acoustid from the file is kept around when a file gets matched to loaded metadata on the right pane.
So scripting is currently out of the equation here Just to verify a few options: Does it work if you delete the tag manually from the files (select the files, right click the tag at the bottom and choose “Remove”)? Does using the “Clear all tags” option work (be carefull with this one when testing, it will really remove all tags and write only the tags shown in Picard to the file)?
If these two things work I would suggest adding a new ticket to request specifically being able to unset the acoustid_id using scripting. I currently don’t know for sure how this can be fixed, but the scripting probably needs to be extended to apply to more cases. Now that Picard actually supports having mutliple separate scripts there could also be some optioin to specify when and on what data a script gets applied (e.g. on metadata loaded from MB, on file metadata when a file gets added, applied just before saving a file).