Database module: How should separate groups work with multiple group membership?

When using the database module, the separate groups function is not working.

Reproduction:

1) Create database
2) Create two separate groups A and B
3) Make a student member of groups A and B
4) Make one entry in the database module (separate groups)
5) Check database entries for both groups

The student's entry will not be visible in both groups.

See also: MDL-47796
Discussion: https://moodle.org/mod/forum/discuss.php?d=283906

(Probably refers to the following discussion instead:
https://moodle.org/mod/forum/discuss.php?d=305356 )