I did encounter the same issue on my Mac today with the Geometry option in Affinity Designer v1.6.1.
When I use Geometry > Subtract when I have to objects selected, it seems that the Geometry process freezes.
The Subtract progress bar shows as expected and shows completed (progress 100%) immediately, but the progress bar does not disappear and after that I cannot continue/ select objects and so on. Only solution is to force quite the application and start it.
The issue occurs every time when I use the Geometry tool.
These are my Mac details: Mac mini, Intel Core i7 3,2 GHz (6 cores), L2-cache (per core): 256 KB, L3-cache: 12 MB, Memory: 32 GB
Same issue occurs in Affinity Photo v1.6.7
I hope you can help me.
Thank you very much for your support.