What make/model of phone?
Is it a Samsung by any chance as they have an annoying habit of spontaneously losing their settings and needing reconfiguration.
Lots of people all around the world have reported this but I have never seen an authoritative explanation as to why this happens or a general solution.
Otherwise is your phone getting regular software updates that perhaps reset certain settings?
Ultimately it's almost certainly not down to the 48 network/service and is more likely an issue with the phone itself.
Hope this helps.
I've seen people have this issue repeatedly, its incredibly frustrating.
I have a theory.
I can't rule out software updates as a possible villain causing the issue on with Samsung devices occasionally, but I know what does for certain, excluding software updates!
Cross the border into Northern Ireland, lose signal, return to Republic, search for signal, APN will have disappeared.
I checked this out on an Samsung Galaxy Note, S4 and S2 all having the same issue.
What Samaung model is the OPs I wonder?
Is it up to date with software?
Are you manually inserting the APN?
Are you selecting save from the top right after inputting the APN?
How long is it since you got your last 48 sim?
From my very basic understanding and troubleshooting of the issue, the above questions are important to solving the problem.