QGIS » QGIS Application

affected version:
occured with 3.4.2
QGIS installed through OSGeo4W

precondition:
make sure, osgeo4w/share/proj folder contains transformation grid (in my case BETA2007)

reproduce:
1) Settings > Options > CRS > "Ask for datum transformation if several are available"=checked
2) load geodata (in my case a shapefile, GK zone 3, EPSG:31467)
3) Right-click legend layer in legend > Export > Save features as
4) enter file name and select a different CRS (in my case UTM zone 32N, EPSG: 25832)
5) click "OK" -> New window "select datum transformation" opens
6) choose one of these datum transformation options (in my case BETA2007)
INFO: BETA2007 is not a paramter transformation but a grid-based transformation
7) click "OK"

expected result:
shapefile is transformed using BETA2007-grid

actual result:
QGIS ignores the selection for BETA2007 and uses some other parameter transformation (the default transformation?).
The same happens when using transformation grid BWTA2017: Both results are identical.
Transformations with grids are much more accurate!

hints:
transformation worked correctly in 2.18.26

attachment:
-shapefile in EPSG:31467 to be transformed into EPSG:25832 (original.shp)
-shapefile transformed wrong (transform_wrong.shp from QGIS 3.4.2)
-shapefile transformed correctly (transform_correct.shp from QGIS 2.18.26)
(Note the difference between the two of about 1.70m!)

why is this important?
Germany is in the process of changing it's national grid from Gauss-Kruger to UTM coordinates.
For this, the governments requires BETA2007 to be used (in some cases BWTA2017)