Battery voltage drops from 12.3V to 11.2V in 5mins

When the battery goes through charge/discharge cycles, the chemical composition of the internal plates changes, and they have different volumes. Eventually the swelling/shrinking will cause little cracks which will eventually connect and allow little fragments of plate material to fall to the bottom of the battery. When the pile of junk at the bottom grows high enough to contact the plates themselves, it's a short circuit (although limited to one cell at a time).

Another possibility is that sulfation has occurred in the plates and the capacity is greatly reduced. A new battery takes a long time to charge from dead to charged, and a long time to change from charged to dead. If you put a battery charger on this battery and it gets a full charge in 5 minutes or so it's probably due to this reason.

If you disconnect the battery when it's fully charged and it still drops off in a few minutes I'd suspect the first reason, otherwise the second reason.

Either way, does the battery get so dead that you have slow cranking in the morning? I wouldn't worry about it until then but when it started cranking slow I'd spring for a new battery.

battery voltage for fully charged is 12.6V

-for starts you only have 12.3V is not fully charge.

-if your battery is old enough (2years or so) it's likely sulfated

-if you have wet type battery please check your battery acid level.

-car heater is usually 1000 Watts or more some model even have 3KW heater. let's says you have low watt heater, 1000W/12V(rated voltage)= 83A with this 83A draining from 12.6V it's pretty much way too much without any additional source (charger)

Let's say you have 60Ah 12V battery is around 720W/h and you draw 1000W (1000w in 5minutes is around 83W/h (11%) ^this calculation alone is far less than real world, because you use heater on cold the resistance in heater coil is pretty much lower before you start use it and usually draw almost 2-3x the rated watt you will close to drawing 3KW and radio is around 5A (Depends on your type and modification with amplifier)

that's why in operating manual all manufactures are stating you can turn on heater after you start the car.

Is this behavior normal, or should I change the battery?

No, it's not normal. From your description, you had two accessories turned on: radio, and ventilation fan (in a non-electric car, the heat comes from the engine, so if the engine is off, you don't literally have the heat on, though you can still extract what heat remains in the ventilation system's heat exchanger by running the fan). That's not enough to drain a healthy 12V automotive battery from 12.3V to 11.2V in only five minutes.

Instead what's happening is that when the battery is charged, that includes "surface charge", which gives the illusion of a higher charge on the battery than there really is. (See e.g. What is surface charge and how does it affect battery testing?). The surface charge is quickly removed with any minimal drain on it.

Unfortunately, once a battery has declined to this extent, it's time to replace it. Even if the starter is cranking well, there's no reliable way to know how much longer it will last until it doesn't crank at all. You could get a gradual decline in performance, or it could go from great to nothing overnight.

If you don't mind being inconvenienced, you can wait until the battery dies completely. But otherwise, it's time to get a new battery.

(Note that many batteries come with a pro-rated warranty, so if the battery failed early you may be able to get some of your original purchase price back, to offset the cost of the new battery.)