Hey,
recently I have encountered a problem while merging a bugfix branch with just one commit. I had the option "--no-ff" checked in Bugfix section in IDE GitFlow plugin settings. I found a similar issue in the original repo regarding merging one commit features:
OpherV/gitflow4idea#225
I looked up what fixed the problem in the original repo for features: OpherV/gitflow4idea@c9c1b73

It seems that the src/main/java/gitflow/GitflowImpl.java file, misses similar line of code for bugfixes:

It is just a quick research so i moght be wrong but it seems right :)
If you need any other information just let me know!