Today I installed the freshly made 11.3 and pulled the onlyoffice connector.
I installed license... no trouble.
I uploaded a docx attachment and clicked on the little stack of 3 squares to edit it. I obtained the screenshot attached.
A dialogue on XWiki chat shows that this change is justified by XWIKI-15450 which was indeed deployed with 11.3.
A potential fix has been requested as XWIKI-16378 however we should also have a fix of the app.