It happens, sometimes more than others, some people never see it, others see it a lot.

It happens because of a glitch with the radio signals between the smart meter and the company requesting the data. There can be many thngs that might cause a glitch.
The meter has the data and keeps it (for the last 30 days I think) but after a number of days the company will stop trying to ask for it, it isn’t really needed as long as they have later readings.

In the end it doesn’t realy matter if a day or more goes ‘missing’ what counts for your billing is what your meter says at (or near) the start of your monthly billing period and what it says at (or near) the end of it.

As long as they have meter readings from ‘near’ the start and end of the billing period then they can estimate the rest from those. (Or can just estimate the whole month if they have no readings at all).

That’s what you will see displayed for the Full Day when there is no half hourly data showing - an estimate.
(If they do get the data later then it will show up and that estimate will get corrected).

If they can’t get any smart readings at all then they will ask you to read the meter and send the readings in yourself, just like you have to if you don’t have smart meters.

It’s easy to send in readings, you can do it online, or in the app, or by phoning support, or by webchat.
However you don’t need to send any unless they ask you to. (Although you can if you want, before I had a smart gas meter I would send the gas meter reading once a week).