I'm not familiar with the issue, do you have Koala set to Auto-compile? You could try Calypso as alternative CSS compiler: https://docs.mendix.com/howto/front-end/calypso#1-introduction. Since it got out I haven't used Koala anymore..
The link to download Calypso within the link above doesn’t work?
I've observed the same behaviour with Calypso after some time has passed, unfortunately