For Read permissions as per your scenario, try clearing browser cache, close all browser tabs, open a new tab (e.g. Mozilla Firefox) in Incognito Window, follow the steps that you did, and while assigning groups, keep on saving them one-by-one.
I wasn't able to reproduce the issue.
1.) Created a user named "testpermissions" .
2.) Create two groups, a.) testauthorgroup (Read permissions to ca & ch) & b.) testdevelopergroup (Read permissions to de & es).
3.) Assigned "testpermissions" user to those two groups.
4.) "testpermissions" has Read permissions to ca, ch, de & es.
testpermissions user groups:
testpermissions user's permissions:
In case of any deny/allow conflicts, as Arun mentioned:
Access rights from multiple group principals are evaluated based on their order, both within the hierarchy and within a single access control list.