Deprecate automatic changes to the tile size with HCOMPRESS_1

astropy/io/fits/hdu/compressed.py

@@ -11,6 +11,7 @@
 
 import numpy as np
 
+from astropy.utils.exceptions import AstropyDeprecationWarning
 from astropy.io.fits import conf
 from astropy.io.fits._tiled_compression import compress_hdu, decompress_hdu
 from astropy.io.fits.card import Card
@@ -1103,6 +1104,8 @@ def _update_header_data(
 
             remain = self._image_header["NAXIS1"] % tile_size[0]  # 1st dimen
 
+            original_tile_size = tile_size[:]
+
             if remain > 0 and remain < 4:
                 tile_size[0] += 1  # try increasing tile size by 1
 
@@ -1125,6 +1128,13 @@ def _update_header_data(
                         "Last tile along 2nd dimension has less than 4 pixels"
                     )
 
+            if tile_size != original_tile_size:
+                warnings.warn(f"The data size should be a multiple of the tile "
+                            f"size. The tile size has automatically been changed "
+                            f"from {original_tile_size} to {tile_size}, but in "
+                            f"future this will raise an error and the correct tile "
+                            f"size should be specified directly.", AstropyDeprecationWarning)
+
         # Set up locations for writing the next cards in the header.
         last_znaxis = "ZNAXIS"