I've looked into the issue and I have a proposed fix. Could you test this in your application and see if this fixes the problem?
The new .mpk can be found here: https://github.com/mendix/Bootstrap-RTE/tree/master/test/widgets
The new version works. Data is passed to the attribute and saved in the database. However.... I still get some errors when I want to create a PDF, using the generated (X)html code. It looks like the breaks < b r > are not correct (should be: < b r / >) and you can't use the ident function when you want it more then one level. the XHTML code. The PDF generator will crash when you use this code.