We have seen that behavior with one of our customers before. That was because of a compatibility issue between the Mendix release, a widget and Internet Explorer.
In some cases text fields and reference selectors became readonly, not always but only after performing certain actions.
We found that it was caused by an combination of an old Mendix release and in our case the rich text editor was causing issues. The only way to know for sure what is causing your issues, is if you can reproduce it at least in your project. This way you can test if it has something to do with the platform or with an appstore widget.
I see that you are using Mx 4.4.4, be aware that this release does not support IE11, simply because Mx4.4.4 was released before IE11 existed. If these issues only occur in IE11 you should start with upgrading to the most recent Mendix release.
For your information, we have never been able to reproduce it in a small test project. Just the rich text editor in a simpel page never gave any problems. In our case it was the combination, of browser, end-user pc, latency, frames, tab pages, etc. that was causing our issue.