Problem with measures

I have a problem and I don’t know why it appears or how to solve it so I need your help. I model some boxes for the client
power supply. It is necessary to make the models in the given measures, millimeters are in question. The problem occurs when I export the model (.fbx) and when I save the blend file. When I reopen the blend file or import the model the measurements are not the same as when I exported or saved. The difference is in a few millimeters. What should I do, how can I solve

this? Everything is on a 1: 1 scale

I cant reproduce it with 2.83,
which version you use and did you change to mm from start of or later?

Blender 2.90.1 From the beginning I worked in millimeters

Ist the scale applied or at a periodical value?
Could you upload an example?

Scale is applied, everything is 1: 1. Unable to share file due to contract unfortunately.

I am afraid you have then to find the cause by try and error.
Can you reproduce this problem if you create a new example?
If yes which version you use, did the problem persists if you use a newer or older version?

I cant find a bug in the database, perhaps you open a new case:

I had to re-adjust the dimensions and export all 14 models. Some I managed from the first time, some I had to repair three times. I barely got to finish the task, and I honestly have no idea why this happened. Everything is modeled in edit mode, everything is on a 1: 1 scale, from the very beginning I modeled in millimeters. I do not know.