Not sure if this was the actual reason but Microsoft have now deployed a fix for our issues and we have tested and the forms are responding fine again. Note: some users needed at CTRL R or CMD R refresh in their browser before they worked again despite closing
and reopening their browsers.
Current Status: We investigated an issue in which users are unable to retrieve a subset of SharePoint Online lists when using a specific Application Programming Interface (API). The investigation determined that an update intended to improve the mobile user
experience caused the API to return incorrect data. We've begun to deploy a fix and functionality will be restored for affected users as soon as the fix reaches their environment.
User Impact: Users may be unable to use custom applications and features that leverage Representational State Transfer (REST) Application Programming Interface (API) calls to retrieve SharePoint Online lists. Additionally, SharePoint 2013 workflows which
retrieve lists may be affected as well. A subset of lists are affected by this issue.
Scope of Impact: A few customers have reported this issue, and our analysis indicates that impact is specific to a subset of SharePoint Online lists.