Image goes transparent

[Ofnuts]

To quote the doc on gimp-drawable-fill:

Quote:This procedure is unlike 'gimp-edit-fill' or the bucket fill tool because it fills regardless of a selection. Its main purpose is to fill a newly created drawable before adding it to the image. This operation cannot be undone.

For the rest, hard to tell without your source code... there is also the possibility that some tool option is set, or the color has an alpha component (Edit>Preferences>Tool options>Reset to default values`).
Python is easier to debug, because you can easily add print statements to get the value of variables...