Setting the WebToggleButton to rounded corners a rectangle corner is also visible.
Is this with the latest version v2.3.2.0 of TMS WEB Core?
I retested this here but could so far, not reproduce a problem with rounded style
After having removed bootstrap the toggle button looks fine. No checkbox behind. Still a bit strange because in a test with bootstrap on and off the toggle looks fine.
If you have steps to reproduce / sample project, please let us know.