sp.stellar_mass variation

[astromariana]

Hello FSPS people!
I hope you're doing well.

I've been exploring various SPSs, and I noticed that in FSPS, the sp.stellar_mass parameter shows some variation in the mass fraction between logage 5 to 6.3. For comparison, I observed a decreasing behaviour in the mass fractions of the Bruzual & Charlot SPS. @bd-j suggested that "is due to some numerical issues in the interaction of the IMF upper limit with the sampling of the high mass end of the isochrones at very early age" on dfm/python-fsps#66, but we'd like to know if it's true and if the mass fractions could be improved.

Could you provide some insights into this?

Thanks in advance for your guidance!