Unfortunately, handling DPI-awareness correctly on Windows is not an easy task and it only becomes more difficult when you are hosting plug-ins which may behave differently depending on the context in which they are opened. The scaling in the JUCE hosting code is based around the scaling APIs that are available in the VST2 and VST3 specs but many plug-ins don’t implement this, or have their own workarounds for rendering at high resolutions which interfere with this so it’s a total minefield. The JUCE DPI-aware scaling code has been tested with a lot of plug-ins, but there are plenty of edge cases and weird behaviour, and it’s far from perfect - we’d certainly welcome any suggestions for how to improve things.
To get the old non DPI-aware behaviour back, you can recompile the plug-in host with the JUCE_WIN_PER_MONITOR_DPI_AWARE flag set to 0 in juce_gui_basics.