Nothing wrong with drawing SLA down to 50% SoC or a fair amount lower.
0% SoC causes damage faster. 30% or 20% SoC cycling would give fewer cycles, but not much fewer Ah of cycling (DoD x number of cycles)
It should be recharged shortly after; don't leave below 100% SoC.
My system is configured to disconnect loads at 70% DoD, completely shut off inverter at 80% DoD. It has SunXtender AGM, and I should get several hundred cycles with these settings.
Your "100%" and "84%" are probably voltage readings translated to SoC?
Of course voltage drops when you stop putting current into it, but if it drops to an indicated 84%, that suggests not fully charged.
Double check SCC settings for absorption voltage and time.
What "C" rate do you charge at, current vs. Ah capacity?
AGM and Gel batteries need different charge voltage (and maybe current) compared to FLA. So your charger settings might not be correct.
The other possibility would be if cells are of different SoC, so they settle to different voltages, some much lower.
That doesn't happen early in properly-charged life of a battery, but develops over time. (Or if abused, left at low SoC.)
How old is this battery? If practically new, I think you could parallel another. But only after charging and observing it has returned to expected voltage. If voltage remains low, then yes it would drag down a new battery.