So it seems that 'normally' during a geyser eruption, you get a period of time where it emits 666g/s of steam, then for a few seconds in the middle of the eruption it emits 50kg/s of water, then it goes back to steam, then the eruption ends. This seems like it is the intended behaviour.
However when the geyser is partially (NOT completely, then the steam seems to not come out due to depressurisation) submerged in water (usually produced by itself), it seems to generate additional copies of the steam/water (this reads as multiple lines of 'generating X steam/water'), and I think it has to do with how many partially covered water tiles there are?
This is what appears to cause the absurd '700 tons of water/steam in a single tile' in geysers that are in a very tight space. It's also what seems to be causing the issues with 'geysers producing absurd amounts of water in general', because if a geyser is in a small space with only a few tiles, what happens is all the steam goes into 1-2 tiles and the water is below, which causes this effect (because the geyser is partially submerged but not completely because the steam has nowhere to go).
I'm not sure if I'm explaining it well, but there you go.
Edited: Seems like it's related to something else, and it's possible it has something to do with reloading the game. Either way, should discard this and look for the issue causing the multiple emissions elsewhere (it was very consistent for me, but I guess other factors were lining up to cause it).
Observe a geyser eruption when its partially covered by water (but not completely) and note that it's producing extra copies of the 'generating steam/water' lines (and actually producing extra steam/water). Seems to happen every time to me, but there might be other factors contributing to this bug.