I’m aware of this issue, as is Audio Modeling and I’m in correspondence with their QA team however since I’m in the middle of a rather large update to Cantabile I haven’t had a chance to look into it yet.
I plan to address this (see here: RFC: Removing Plugin GUI upscaling) as part of next major update and depending on what I find, I may retrofit any low-risk bug fixes back into the current series.
As a temporary work around try experimenting with Cantabile’s HiDPI modes → Tools → Options → General → User Interface → HiDPI User Interface as I believe this can help. If you find a combination that works, please post back here to help others.