The ispindel thread is this one
https://www.homebrewtalk.com/threads/ispindle-diy-electronic-hydrometer.598187/
notice the different spelling hence more tricky to find.
But are you using ispindel software or gravitymon? Have you recently updated or has it always shown this " raised " abv figure.
My first thought is the formula, I'm not a user of brewfather but use my ispindels either with brewspy or brewfather.
Put the ispindel in config mode using the little reset button, grounding pins etc. Log into the ispindel network and look at the configuration screen, make sure that it has been configured on a flat surface ( battery down ). This sets the 90 degree angle.
Vertical will then be 0 degrees.
See if this changes anything.
I'm not sure if brewers friend works out the angle and formula or it uses the formula provided by the ispindel. I'd suspect this is where the issue is or there is a set of units in plato that are wanted in SG is another consideration.
You may have checked these things already.