When changing editor shortcuts / input settings, pressing the AltGr key will cause the editor to read the key combination of Ctrl+Alt rather than a single key press.
This is likely due to Ctrl+Alt being the shortcut for AltGr behavior for keyboards that don't have an explicit AltGr key.
Create a blueprint with 4 input nodes, ctrl/alt left/right
Connect each one to a print node that uses append and get key display name to output pressed/release <key>
With english set as keyboard language press ctrl/alt left/right and see expected outputs
Change keyboard language to French - Belgium
Press ctrl/alt left/right and see that when pressing right Alt you also get Left control press/release printed to screen
Press and hold right alt (note right alt and left control pressed output), then press left control. Note that Left Control Pressed is not output.
Release Left Control and note that Left Control Released is output.
Release Right Alt and note that Right Alt Released is output as well as Left Control Released again.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-48588 in the post.