On Linux, pressing "Ctrl" changes my para to RTL

This seems to be a new problem, as I didn’t use to see it.

Using LibreOffice on Linux Mint 17.3 (Cinnamon), have RTL enabled in complex languages (Hebrew)

Editing an English LTR document, if I press Ctrl, for example as a lead-in to Ctrl+B, the paragraph direction quite often changes from LTR to RTL.

The system hotkey to switch keyboards is set to L-Shift + R-Shift.

I don’t have this issue running on OS X with the same version of LO.

Does anyone know how I can prevent this (very annoying) keyboard problem?