Solved! Go to Solution.
I had a similar problem and first tried to convert my data as well. But, than noticed my Geotiff had a compression applied ('PACKBITS'). After exporting my dataset (right click layer =>Export Data) to an uncompressed Tiff (Compression type: NONE), the problem was solved: NoData cells behave again like expected. So have a look at your layer properties to see if your Tiff has been compressed as well and if so uncompress it.
Good luck and best wishes,