Using the better MC modpack 1.16.5 forge. When hatched, baby dragons do not show a normal growing time - it stays at 0 seconds.

When giving food to heal and trying to make the dragon grow faster, it doesn't actually grow faster: it does not grow at all. The dragon can be healed.

This seems like a quirk with the modpack your using, as dragons grow fine in 1.16.5. Unfortunately that game version is really old and I don't see it worth exploring anymore, so I won't be investigating this further. I apologize for the inconvenience.