The DVK is a weak frame TBC, but it usually removes anti-copy (artificial video errors).
A main "issue" (user error + undocumented feature) is that you can wire the unit wrong. It has both a route that passes through TBC, and a route that bypasses TBC. You've hit the bypass.
Best of all, the DVK varies. So the engage/bypass isn't always the same. There's at least 3 variations, but may be more. The TBC-1000, for example, has at least 6 versions, and some apparent subversions. I often see "new" TBC-1000 hardware combinations, meaning DataVideo probably used whatever was available, and made on-the-fly changes to units to maintain integrity -- something that failed for Cypress, hence the bad "black" era 2010s units.
Some settings may also cause bypass.
This is a complex unit, and lots of effort for a mere weak TBC. It costs less money, but it costs more time. This is why I preset units, lock it, and block ports that shouldn't be used.
Each channel has in+out. Sometimes out engage, but it usually bypasses.
BG always bypasses.
The main output usually engages, but not always, I've seen it bypass (and use channel out instead).
And again, none of this is documented. DataVideo was really bad about documenting how their units worked. If you ever take apart their gear, you'll notice jumpers, DIPs, and pots, none of which have documented instructions. I had to figure all of this out by experimenting, risking the pricey gear. The key is I knew what I was looking for, something most newbie users do not.
BTW, ES15 is a DVD recorder, not a TBC. Yes, it has a line TBC, but the distinction must be made. The TBC is crippled, it has "holes" bored into the TBC function, to allow anti-copy. It can cause the ES15 to trip up. And those errors would then be passed to the DVK. Anti-copy is not always like a natural error, though still artificial.
Also don't forget that condition matters. These units are now all 10+ years old, and can be defective. So a random
eBay buy can quickly turn into a paperweight buy, especially since it's often 30+ days later that you start to realize problems. This is a huge issue with VCRs and TBCs, when you're new to all of this gear, and don't really yet understand what's going on.