Poly(3-hexyltiophene-2,5-diyl) (P3HT) reactions with inorganic salts in aqueous solution
I'll point to a very recent paper on oligothiophenes for solution-gated FET sensing that suggests there are no chemical reactions. (This article popped up in my feed - there are many others on solution-gating organic electronics.)

$\alpha,\omega$-dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors. Appl. Phys. Lett. 108, 073301 (2016)

The authors look at many factors influencing the device performance:


  Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications.


The devices in this article use KCl and phosphate buffer. They vary the pH and ionic strength.

I haven't seen anyone suggest chemical reactions, and I'm not sure how it would occur. Certainly the oligo- or polythiophene could degrade over time, but most articles argue these are stable for long-term use.

The authors do note a small drift on the time scale of days:


  both changes may possibly be attributed to the passivation of trap states at
  the semiconductor-electrolyte interface by water molecules penetrating into the thin film


I can't speak to the drift you see, but I'd make sure that:


You aren't seeing evaporation of solvent over time (e.g., from device heating) that's changing your electrolyte concentration.
You use a proper reference electrode
You try removing water via vacuum and repeat the experiment (i.e., you drive off any water penetrating the semiconductor film)


Certainly organic semiconductors are porous, and while mostly hydrophobic, it's not unreasonable to expect that after days of use, some water may migrate into the film.