This is a known issue (UE-4659) it has been submitted to the developers for further consideration. Here is a link to the original issue. You can go here for updates.
I have the exact same issue with a custom sized canvas panel cropping off a line of text that I want to rotate vertically. I’ve tried all sorts of clever workarounds but no success to get this to work reliably.
It would be useful to have a horizontal or vertical orientation control similar to the slider or progress bar. Or secondly, a control on all panels to control clipping of its child objects. Or perhaps just, fix the way render transforms work, so at least there is a method that will work even if not so elegantly. (I really dislike how the bounding box is oriented differently than the render transform, its hard to work with as well) I vote for all three options, allowing us to control more completely how we set things up.
I’m glad this is a known Issue though! Thanks for your help in advance!