Ok. I tested. I use 2 widgets, one is my own widget with many imageviews and canvas, and one is eral's widget from the tutorial.
After restart the phone, if I run Erel's one first, it works well, no bug. Then I follow to run my widget. Now bug appears. After that, again I run Erel's one. Now Erel's one also has bug
After that, the bug seems to remain there any no matter which widget I run, the bug is there. The only way to get rid of the bug is to restart the phone.
So, I think the problem here is, once a widget with many imageviews and canvas are loaded, OS or B4A can't handle it and the memory somehow getting bug which cant be recovered until a restart.