SWF Assets - not correct rendering

I have swf timer which use in my project. On flash target everything looks ok , but when try use on html something wrong happen. Here is the simple project:

Hmm, it seems like it may be related to masking. openfl test html5 -Dcanvas looks a bit better, though there’s still a section where the mask or the rendering seems off

Where I can check source code for masking ? Is it easy to resolved the problem or is connected with missing functionality for masking ?