I wouldn't say this is a bug per se, but more the fact that these two pieces of software do not seem to play well together.
Steps to observe the problem:
- One must have both Grammarly as well as BetterTouchTool installed.
- In some apps, when using a Single Finger FORCE Click using Apple's Magic Trackpad, I do not get the expected behavior, "Lookup Word Under Cursor", which is a gesture I have defined in the BTT for all apps. The problem presents itself mostly when composing new e-mail messages in MS Outlook for Microsoft Windows (Yes, in a VM that I run using Parallels.)
- Instead, the word under the cursor is selected and no lookup is presented. And, when I release/lift my finger the entire paragraph ends up being selected.
- If I disable Grammarly, the gesture works with no issues, even this being a VM under Parallels.
Yes, I do understand this issue may be out of your control since it involves another third party app, but if we come to think of it, BTT does interact with several other third-party apps... So, some effort should be used towards making BTT compatible with what's out there (within reason, of course). And it might as well include Grammarly. 
As I said, this may be out of your control, but, if it is something you can relatively easily rectify, that would be GREAT!
Thank you so much for all the hard work that you put into this awesome software! Best, Raphael