This is unfortunately a limitation of the TTMSFMXWebGMaps / TTMSFMXWebOSMaps components. The components are using a transparent dialog technique to show a native webview component. This is shown on top of all other controls. There is currently no workaround for this issue, but will investigate if we can find a workaround.
Sorry, this is a technical issue, the TTMSFMXWebGMaps is a native control that lies on top of all other non-native controls such as the TTabControl / TComboBox which uses FireMonkey framework rendering. There is currently no workaround for this issue. TWebBrowser has the same issue with the only difference that it is managed by TTabControl, which removes the need to toggle the visibility manually. Unfortunately we cannot intercept this event and thus cannot toggle the visibility of the control when a tab changes.