GetThumbnail doesn't seem to work as advertised

Issue report by Kristof Gypen

Product

FileMaker Pro

Version

13

Operating system version

MacOS 10.10

Description of the issue

In the test version I was using at the time i created a little photo management tool in FMP12.

That was possible because of the new GetThumbnail container function.
The script involved, creating a path from user inputs, using that variable to insert the picture into a container field (which uses Secure Storage).
And then rescaling that image to some default width and height using this script step:
-> Set Field [Covers::Picture; GetThumbnail (Covers::Picture; 250; 350 )]
to basically replace the loaded image with a smaller version.
That had a nice undocumented but wanted side effect that it also changed the resolution to 72dpi.

Now in FMP13 this last set field script step doesn't seem to do anything, although i believe it should.

Steps to reproduce the problem

see above

Expected result

image should be scaled

Actual result

image stays the same, when exported using Export Field Contents

Exact text of any error message(s) that appear

no error message, just not the desired action

Configuration information

field definition is Container - Secure storage

Workaround

none found, should go back to version 12 which i don't have.