The first few albums before Nothing were a 4 string tuned BbEbAbDb, with a 5 string on Chaosphere tuned the same. Nothing onwards they switched to BbFBbDbGb for everything to be in the same octave as the guitars with an added low Bb. Listen to the Obzen title track, it jumps from a low Bb to an F at the beginning and confirms this. However the song Spasm from Nothing is in some ridiculously low G# tuning or something which is only used on that song I believe. On Bleed from Obzen the main riff is in Eb so he uses a 4 string tuned BbEbAbDb for that song- this was on one of the gear rundowns on YT. Play along with anything post Nothing (Catch 33 especially) and the drop Bb tuning will make sense- almost all the riffs are written in F on Nothing for example.
I've done a lot of research on Meshuggah and I remember you posting a similar thread a while ago to which I responded similarly