Currently, HTML web resources are not supported in offline mode. But to add some custom UI on the form we need the HTML web resources functionality. However, due to these limitations, we are unable to add HTML web resources to the form.

Microsoft has mentioned in docs that HTML web resources are supported in offline mode but it is not supported yet in offline mode.

Earlier in the field service mobile app HTML Web resources were fully supported in both modes as well as on all platforms. But In this new field service(Dynamics 365) Mobile app HTML web resources are not supported even after adding just a simple heading tag.

Requesting the Microsoft team to work on adding support of HTML Web resources offline for the field service(Dynamics 365) Mobile app same as supported in the old Field Service Mobile app.