Is it normal to hear the electrolyte flowing or bubbling when charging an AGM SLA 12V battery?
The symptoms are bad and you should have stopped immediately. 

Based on max charge energy of 20Ah or 260Wh on a full battery there be electrolyte loss, but hard for me to estimate how much. Although your capacity is 260Ah or 3.4kWh that is only 8% overage, when it cannot store more charge ! It boils electrolyte.


Normal maintenance test is to measure specific gravity if possible. 


Next plot discharge rate V vs time for say C/10 constant load Amps to get capacity from specs for Voc vs SoC. (V open cct vs State of Charge). 
But do not charge and load at the same time unless it is at float V



To me, it indicates the charger was not so smart at detecting a weak single cell and over-charged that weak cell causing H2 outgassing from a weak cell which appears to have been sulphated.

From the web I found this;     

The 8 stages are:
1. Desulphation
2. Soft Start
3. Bulk
4. Absorption
5. Analyze
6. Recondition
7. Float
8. Pulse     

What historical performance and age can you tell us?

From my experience on pulse desulphation designs that work,   they got the sequence wrong.

I would have designed it to quickly sequence as follows 2,5,3,5,7,5 then if dV/dt steps too much with pulses, that indicates high ESR from sulphation and then continue with 2 until it improves applying 7 and 2 simultaneously.

Write to Mfg and complain with your test results and answer to above.

Although I don't know your battery specs, similar ones are below