This has been reported to Google Support by users and already escalated to the concerned Gmail team. The team wants relevant details specific to this issue, so for others reporting the same issue be sure to submit feedback directly to the concerned Gmail team via the Feedback option in the Gmail Android mobile app. This process also includes relevant device, browser, app, and OS details as well to best help the team. The team is known to read all feedback submitted to them, although they may not respond back unless they desire additional information related to the topic of your feedback. The purpose of sending Feedback is to also give the team a measure of the volume of users being affected by an issue or requesting a particular feature.
As I said previsoulsy, reinstalling the App appears to provide a temporary fix. Alternatively, after providing Feedback to Google via the App->Feedback option, you can revert to a previousl (working) version via Settings->Apps->Gmail->Uninstall Updates. Hopefully Google will provide a more permanent fix in due course.
