A little update:
I tested the following games:
BC version 2.00, AGI 2.439
KQ1 version 2.0F, AGI 2.917
KQ2 version 2.2, AGI 2.917
DDPG version 1.0C, AGI 2.272 (Donald Duck's Playground)
LSL1 version 1.00, AGI 2.440
SQ1 version 2.2, AGI 2.917
Of those games, all with an AGI version lower than 2.917 (i.e. BC, LSL1 and DDPG) display the faulty behaviour.
Also funny is that DDPG asks for joystick calibration when selecting auto or Tandy, but doesn't when selecting cga, vga or hercules. Also, DDPG doesn't seem to support Hercules, and defaults to CGA.
My provisional conclusion is that DOSbox emulates some Tandy registers in auto mode, to trick the older AGI games into believing Tandy is present, without fully switching into Tandy-compatible mode.
IMHO, Tandy should never be "auto detected", since it never was part of normal PCs. I think DOSbox needs better machine configuration capabilities, being able to select basic machine type (PC, Tandy), and subselect all other types of hardware (e.g. processor type, graphics card etc.).
JAL