Microsoft says that Bluetooth speakers will stop connecting to devices running Windows 10, version 1903 after installing the KB4505903 cumulative update released on July 26, 2019. Users will also notice a Microsoft Bluetooth A2DP Source entry under the Sound, video and game controllers node displaying a yellow exclamation mark icon in Device Manager. This hints at some issue related to either missing drivers or the driver not being installed correctly. Microsoft also provides a workaround for fixing this problem using the System File Checker tool to repair corrupted or missing system files.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-kb4505903-update-breaks-bluetooth-speakers-connectivity/