Normally I would say the GPS antenna is faulty / weak / dying, and recommend a new GPS antenna.

Note that in your two pictures you have 7 minutes time difference between them, and as GPS satellites orbit the earth, the signals wax and wane with time.
I see satellite 9 disappeared completely, but satellite 26 only dropped from 15.8 to 14.0
And satellite 27 increased in signal from 4.0 to 12.0
Typical indications that the signal is varying as the satellites move along their orbit.

But if you tested the Mk4 in two vehicles, and the problem stayed with the Mk4, then the GPS antenna must be OK. Please ensure that you have the suspect Mk4 installed in the other vehicle for a few days, to be certain that the problem lies with the Mk4.

So if you are confident that the problem is with the Mk4 unit itself, then the problem must lie in the internal GPS receiver module inside the Mk4.
These are very easy to replace, you can buy a new GPS receiver here:
Navigatie-onderdelen.nl | Navigation par - GPS
It's the item labelled as "GPS Tuner"

No special tools or skills required to replace the tuner . as long as you're not a Clumsy Charlie or a Hulk Hogan and treat the electronics with respect and feeling :-)