Buffer pool never shrinks anymore since update from 10.11.12 to 10.11.15

The following picture essentially tells the whole story for us:

You can clearly see that the shrinking of the innodb buffer pool only triggers/happens during reboots / restarts of the service only since we installed 10.11.12 or higher patches in 10.11.x version (at least up until 10.11.15). And it clearly seems to coincide with the installation of 10.11.12 on our systems.

If these versions are fully binary data files compatible (I didn't do the research on it yet) I can see us even consider going back to 10.11.11 right now.

Please let me know what I can do to help out getting a fix on this and I'll see what I can manage to get on extra information about this.