reset of display scale in "displays"

[AYY]

Ubuntu 16.04.4 LTS

installed unity: 7.4.5+16.04.20180221-0ubuntu1
(not sure 100% that the bug in Unity, but the bug start to appear few days ago)

I have built-in standard notebook display and external hiDPI 2k display connected through hdmi. I use only one display for a time, notebook or external (I disabled notebook display through "displays" for the configuration when external display is connected)

Previously: About year ago, I set scale 1.5 for external display. All this time all works ok: step 1) if I need to change display (e.g., disconnect external one), I sent notebook to sleeping mode, unplug hdmi cable, turn on notebook, and work with scale 1. step 2) If then I sent notebook to sleeping mode, plug hdmi from external display, turn notebook on, the scale is 1.5 for external display, as it was set previously.

Now: step 1) works as usual; after step 2) I have scale 1 for external display, but it must be 1.5 (previously set). So, I need again set it equal to 1.5