Hello Sim161,
Thank you for using Apple Support Communities. Based on your post, your text replacements disappeared and continue to disappear when you create new ones. We'll do our best to assist.
Do you happen to have any other keyboards added to your iPhone? If so, remove all third-party keyboards, Restart your iPhone, and then attempt to add a text replacement again: Add or change keyboards on iPhone
Do you have any other third-party apps that replace text? If so, you'd want to remove these as well, then test it out after restarting again.
Should it persist, what happens when you add the text replacements? Do you ever receive an error? If so, what is that error exactly?
Wishing you well, cheers.