I can't seem to get any GRASS 7.6.0 algorithms to run inside QGIS 3.4.4 Processing toolbox on Arch Linux (kernel 4.20.8).
I have GRASS installed at /opt/grass. Launches fine standalone with tools working and such.
I have gdal and gdal-grass plugin 2.3.2 installed.
ogrinfo --formats | grep -i grass OGR_GRASS -vector- (ro): GRASS Vectors (5.7+)
I compiled QGIS with the following GRASS cmake options just to be sure even though the first compile without them says it found GRASS 7 in /opt/grass regardless.
-DWITH_GRASS7=ON \ -DGRASS_PREFIX7=/opt/grass \ -DGRASS_INCLUDE_DIR7=/opt/grass/include
And inside QGIS Settings > Options > System (tab) > Current Environment Variables there are some bold variables:
GISBASE = /opt/grass GRASS_PAGER = cat
The GRASS Plugin works where it can open mapsets and run tools. QGIS Browser can see and load GRASS Datasets but Processing won't launch GRASS algorithms.
I found a 3 year old bug report filed by myself at https://issues.qgis.org/issues/13072 but I'm not sure if it's a regression and needs to be re-opened.