COM VSTO Word add-in not appearing in the Ribbon

Hello,

I have a client who needs to deploy SAP Data Panel add-in, which is a VSTO developed COM addin for Word.

When I install the add-in (vendor MSI or their setup.exe, from 2013) - installation looks good.
In Word Options\Add-ins - I see the add-in under "Active Application Add-ins" as a COM Add-in type.
No errors in eventvwr, the Word splash screen shows the add-in loading (very quick, so no timeout etc).

It just doesn't appear in the ribbon...unless:

1. I go to Word Options\Add-ins, manage the addin, uncheck it (hit OK), do the same again but check the checkbox to enable. Then it appears in the ribbon.

2. Customize Ribbon. The add-in is already in the right hand pane and check-box checked.
If I create a new tab or new group, then the SAP addin appears.
I did try moving up the order of the addin, but that didn't help.

I've checked that LoadBehavior is 3 in the relevant reg word\addins key.

Situation is the same on Win7 and Win10, and I've seen the same behaviour on 1803 + 1808. 
Currently testing on Win10 1809 17763.107 + 17763.864

It's as if the Ribbon is missing a refresh, but I see no simple way to do that.

Ideas?