Bought a GL703GM-EE101two weeks ago (same spec as your DS74, guess code different maybe for European model, bought in Poland)
General day to day use all seemed fine, including running the latest version of Archicad with reasonably intensive 3D work, minimal rendering work, mostly core intensive work than GPU intensive, bought the laptop for the 6 core processor.
I set the battery health charging for the 60%, and updated all drivers, including the latest nvidia driver yesterday, and was very happy with the laptop until I decided to have a try at gaming seeing as it's marketed as a gaming laptop. After running counterstrike global ops for 10mins via steam up popped the same issue you are having, battery starts to discharge while plugged in. Seems to discharge at a rate of about 7% an hour if connected via a LAN cable and I've came all the battery saving options selected which obviously drops frame rates etc and make game play not the nicest.
I checked the voltage coming out of the adaptor and it's in the range of the 19.5v. Supplied amperage unfortunately I couldn't test. My first thought was that not enough power being supplied by adapter.
I've also uninstalled the batteries in device manager and restarted the laptop for fresh install.
I wondered if undervolting would be a solution but no idea actually how to do that and how it affects the warranty etc.
Unfortunately this is my main work machine being a replacement to my last laptop that fried itself so sending off for an RMA effectively means I'm out of work & income while it's away, not something I'd look forward to. I'm going to head to the shop I bought it off today to see if they have any comment. In the overall scheme of things it doesn't affect me for 99% of my daily use of the laptop but I'm concerned about long term effect/lifespan of laptop. Will update on how I go at the shop.
I've attached an image of battery notice, CPU & GPU results after 10mins playing counterstrike.