QGIS » QGIS Application

hi,
I met a problem of precision of my map composer's scale on different version. Always not solve on v3.2.
After making many tests, I found this.
There is always an error on the composer scale, not big but several tenth of percent. The more my map is large, the less the error is. With round numbers, error is very small and sometimes null but when I increase manually map width (size and position pannel) and tend to the upper round number, error is reaching until I exceed it. At this time the scale map factor automatically change (in main property pannel) and error is again at its minimum. If I continue to increase width map, error is growing until I reach following round number, etc... And error is proportionnaly bigger for little map width.

For my tests:

map width -> error
1 mm 0%
1,99 mm 99%
2 mm 0%
2,99 mm 50%
3,99 mm 33%
9,99 mm 10%
298,99 mm 0,337%
1000 mm 0,1%

My conclusion is that the error is due to the difference of format number between map size (float) and scale factor (integer).
Maybe it's wrong but the facts are here and it's a bit annoying when we need precision.