Bed suddenly deops at around 10mm for couple of microns

Hi, I have an problem which suddenly arises out of nowhere from last few months that print bed suddenly drops for a couple of microns exactly at around 10mm in every print. I am using Marlin and a custom build printer like a Ultimaker where bed goes down for positive Z values. First I thought it was an lead screw or bearings issue at that specific place, so I added a set of more springs and now bed is zero to nozzle but tnut and bearings moved downwards because of extra spring added by around 12-14 mm and I thought it will solve the issue but still there is same issue bed exactly drops at around 10mm...!! 😕😕 I also recompiled Marlin 2.0 without mesh bed levelling and fade height but its still there... Can anyone help with this I don't know what to do next