I don't think it is a problem with the battery, but with the continual trickle charging. It might be worth checking out the set point voltage at the "charge complete" mode on the charger.
Below is a link to an article by NECA on the dangers on using a battery designed for automotive cyclic use in a standby application. Note that having a battery under constant trickle charging is the same as having the battery in "standby application".
https://www.neca.asn.au/necagroup/co...ery-explosions
Specs for the nominated battery charger are here.
https://www.ctek.com/au/battery-char...24v/xs-0-8-aus
This link is to the recommended charging voltages by Century.
https://www.centurybatteries.com.au/...y-charging.pdf
The information you are after is the table at the bottom titled "Maximum Charging Voltage by Battery Type".
Important note: There are two applications; absorption charging and float charging.
If the battery is under constant trickle charging - select the float charging values.
My read of the manufacture's instructions is that the suggested float charging voltage is
13.2V to 13.5V.
The charger's manual suggests that it is holding the battery at
13.6V (or thereabouts) at the conclusion of its charge. This appears to be above the battery's recommended float voltage charge.
Also note that there is nothing "wrong" with the battery charger. An output of 14.
4V when charging is smack in the middle of the recommended range of an absorption charge.
Here is a link to another article that might be of interest on charging voltages for batteries that are effectively in storage.
https://www.victronenergy.com/upload...w_it_works.pdf
I think it is easy to get drawn into the argument that the battery should be stored at 100% capacity. In reality, all you want is enough juice to turn over the engine so that the car starts. At healthy battery at 90% capacity will start all but the most recalcitrant engine.
Some possible options include;
1. Applying the existing battery charger, say, one day per month.
2. If the charging voltage is outside the "float charge" range, dropping the charging voltage down.
Just some things to check out ...