Yeah, definitely a bad thing, since VCAs are what's used to control amplitude levels. Exponential VCAs tend to be used with audio signals, while linears work well for both audio and CVs, although to process CVs the VCA has to be DC-coupled. AC-coupled VCAs are only for use with audio signals. VCAs are one of the essential parts, basically.
Here's an idea: swap out the A-138 mixer for a Bubblesound VCA4p. This fits in the same space, but it gives you four VCAs, switchable between linear and exponential, with mixing capabilities plus the ability to use the VCAs independently of the mix function. Since you have a minimal amount of audio paths here, you could jumper this to be a two-input mixer on VCAs 3/4 and have VCAs 1/2 set up as independent VCAs for processing CVs. That way, you retain your mixing function AND get VCA control over CV modulation.