Sharing to groups and individual group members

Nextcloud version: 16.0.3
Operating system and version: Ubuntu 16.04.6 LTS
Apache version: 2.4.18
PHP version: 7.1.31

I recently stumbled upon an issue regarding combined sharing of a folder with groups and individuals users. I don’t think this is working as expected.

I have a folder shared with a group as readonly. One member of that group needs write permission to one subfolder of that folder. So I shared that subfolder additionally with him with write permission. (Previously this worked as expected; I guess the update to Nextcloud 16 broke it.)
He does not gain write access to that folder. Instead, the readonly sharing of the main folder to his group seems to somehow override his write permission on the subfolder and he can only read there. Giving his group write permission to the subfolder didn’t change anything. The only possible solution was to give the group write permission on the main folder which ist not a feasible alternative.
Sharing with users not a member of that group works as expected.

Is this a known bug (or feature?) or can someone help me resolve this?
I can supply logs if needed.

Is this the first time you’ve seen this error? Y

Steps to replicate it:

  1. Share a folder readonly with to group
  2. Share a subfolder of that folder with write permission to a user of that group