Mobile does not warn when exporting a smart preview instead of raw/full

In LrMobile, one of the available info overlays clearly tells you if you are using an original or a smart preview, so maybe you should turn that on when exporting so that you would know what you are working with. However, if the original of the photo being exported IS available in the cloud, (i.e. not a smart preview synced from LrClassic) but only the smart preview is currently stored locally, then LrMobile WILL automatically download the original IF the export setting "Dimension" is set to "Largest Available Dimensions" OR set to a pixel value greater than 2560 pixels (which is the size of a smart preview). Provided of course that the network is available for that download.

However, IF the device that you are using is currently offline, then LrM will go ahead with the export if the Dimension setting is "Largest Available Dimensions", i.e. the smart preview is used by default, with "Largest Available" being the uncropped 2560 px size. If, however, you have set a specific pixel dimention in excess of 2560 pixels, LrM should issue a warning message stating that the export cannot be done, but that didn't happen when I just tested. That's either a bug or an intended recent change of behaviour.

As Volker says, check your Export Settings (using the icon below).

One other question is that could the image be synced from Lightroom Classic, if you use that?