Staging Analysis Combined ISP is incorrect

source for the formulas used in my calculator
https://wiki.kerbalspaceprogram.com/wiki/Specificimpulse
and my calculator for reference
https://instacalc.com/56249
simplified ISP formula
F thrust = g0 * Isp * m flow
https://en.wikipedia.org/wiki/Specificimpulse#cite_note-11

Please explain how it reports 351 in this case
https://www.simplerockets.com/c/uaQPI8/ISP-bug-demonstrator-50kn

@Pedro https://www.simplerockets.com/c/uaQPI8/ISP-bug-demonstrator-50kn
here is a craft with SRB and efficient engine on 1st stage. Both outputting 50kN of thrust. Mass flow is 24.2 and 13.9 kg. ISP's are 210 and 367. In-game calculator reporting 351 ISP. well it should be 267 in this case. 100,000/(38.1*9.81) = 267 ISP and not 351.

@adonismyth the combined Isp in that source isn't doing a harmonic mean, it's just doing the total thrust divided by the total fuel consumption per second times Earth's sea level gravity, which is what the game does

@Pedro What I mean by the proportional is weighed harmonic mean. I have this calculator to correctly give out dv of stage 1 when multiple engines involved.
https://instacalc.com/56249
It can give out the correct ISP of stage by using 2 different formulas but the in-game one is reporting incorrectly.

It isn't proportional to the thrust but to the fuel flow of each of the engines. I have made this demonstrator to push things to the extreme and show the effect.
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The craft has two engines in Stage 1, one with 12s of Isp at sea level and the other with 190s. The low Isp engine has almost no fuel flow and the large one has a lot, the reported Isp for the entire stage is 190s.