I think you are using version 2.x of the Polyphone software. A bug in this release causes this kind of problem.
Solution: If you like, use 1.9.
2.x ;bug 1:
https://www.polyphone.io/o/en/forum/supp…e-of-polyphone-2-0-12.x; another bug :
https://www.polyphone.io/o/en/forum/supp…mouse-button-release