Check tds before merlin and if it's coming out at about 20, this is about right depending on tap pressure. So problem must lay with Di vessel.
I connected up to a tap the other day and the good water came out at 103 (one hundred and three ) I disconnected rite away I then tried a tap that I use regular and it was normal, so it can be hit and miss if ur not setting up the right pressure, connections, or have any leaks.
I am happy with 20 tds or less from the merlin at a decent trickle and I use about 700-1000L a week and my resin lasts forever.
I have recently been using my tank for pressure washing a lot so have had to rinse tank through with pure going through hose and back into tank, using pole hose end, I recon it was about October when I last changed my resin and I use an 11L Di bottle in van.
My tds shows 002 today as am cleaning tank and changing resin.
I don't know much about wot could go wrong with a Di bottle other than the top screw in part or a cracked bottle.