Wecome Peter.
I’ve not experienced Thick Brisket cut in small pieces. Briskets are usually separated into the Flat and Point retail cuts and then cut smaller for sale. I wouldn’t consider them particularly thick cuts.
You will discover that relating thicknesses details rather than weights will assist you in obtaining more helpful information on SV cooking results from this Community forum.
My first thought was that was a lot of heat pumped into those little chunks of 36-hour cooked meat for 15 minutes. Have you done that before this unsuccessful cook?
Also have you previously enjoyed Medium-Rare cooked brisket? It would have a significantly different texture than the same cuts cooked conventionally, more often Well Done, and could be considered “odd”. whatever that is.
I equate “slimy” to spoiled meat and that probably wasn’t what you tasted because you would know it by now.
Improperly aged meat can develop a taste i describe as funky. That’s not quite “bad”, or disgusting, - and certainly not in the Motown sense of Funk which is delicious listening. It most often happens in Croyovac packaged subprimal cuts kept too long. The meat looks alright but just doesn’t smell or taste right. Unscrupulous meat cutters will heavily salt and rinse those cuts and price them for a quick sale.
Finally, was it too tender?
Or maybe not tender enough?
Some more details could help prevent you repeating the same disappointment.