Andrew,

Related to this forum topic with the attached file as example.
Tested on Win7 32 bit.

Open drawing as is.
Try:
- Drawing Preferences .. Layout .. Viewports .. Four: Equal >> OK
- Menu File .. Revert >> Acknowledge warning dialog: All changes will be lost!

There are still 4 GUI Viewports visible.
Check Drawing Preferences .. Layout .. Viewports >> Reports one single Viewport.

OK won't update to a single as no changes are assumed.
One can fix the current state by selecting another configuration and then a single again.

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A difference spotted against v3.27.6.0 is that the border of the active Viewport turned dark blue before. This is not seen in the latest stable release.

Regards,
CVH