iphone randomly deletes contacts

I got a new iphone SE from apple earlier this year because the previous SE became unstable. Worked well enough for several months and then suddenly deleted about 1/3 of my contacts (at random). But it didn’t stop there… continues to this day randomly plucking away contacts.. used one contact last week, now it’s gone. I’m on 17.5.1 and have stopped updating the ios on my iphones for years (the battery degradations and additional bugs just aren’t worth it). Anyone know the cause of this and even better, the solution? Updating the ios results in a massive battery degradation (which happened on every iphone i’ve owned since 2015, literally overnight my battery goes from working like new to about half) so that is not acceptable (trading one problem for another). This problem seems to be mentioned a number of times over the past few years but no solutions are listed…?