If it is the same as the A53, under settings, general management, there is an option for personal dictionary and the keyboard settings. Hope this helps.
How do you add or edit entries in your dictionary? On my previous phone I used to be able to double-tap a word and choose to add it to the dictionary. That option isn't available on my A52s. Can you do that on your A53?
I use the Gboard keyboard for mine haven't used the Samsung one. But I think if you want to add a specific word to the dictionary you have hit the plus key in the personal dictionary settings. With Gboard it offers 3 suggestions when typing and you can use one of the suggestions by choosing it at the top of the keyboard.
The functionality you're describing is called text shortcuts, and predictive typing. It's not the custom dictionary. How do you delete a word that predictive typing has picked up?
I believe it will only add the word if you choose it from the list of 3 suggestions. If you type a word incorrectly by mistake you can use the back space key on the keyboard to delete. For me Gboard normally remembers word suggestions I have clicked on before e.g. postcodes etc.
