Could be several causes but if configured as you said then it's setup incorrectly causing your issue as the cation output will be higher TDS than input normally.
The electrical charge of the water exiting the cation stage will be higher than that entering. This is due to the cation resin producing hydrogen (H) as a result of the ion exchange. This water will have a higher electrical charge, hence higher TDS on the meter which is just measuring electrical conductivity. The reason the the anion is designed to be after the cation is the anion will add O to H producing pure water and removing most if not all the charge. The mixed is a 'polish' as just two stage isn't a complete removal. This is why normally people will replace anion resins much more often than the other two (in 3 years I've replace anion 4 or 5 times, and haven't touched the cation or mixed).
I haven't discussed not having RO. You maybe lucky and have exceptionally pure well water. If not, then you'll be going through DI Resin much more quickly without the RO.
I have most of my information from BRS here.
BRS RODI