I already mentioned it in a post a few weeks back, but now I have also found confirmation of my findings: a Beverage On Ground (BOG) loses its performance when it lies on soily ground.
In an article (https://rudys.typepad.com/files/qexjul-aug-2016-bog.pdf) written by N6LF and published in QEX, this conclusion is also drawn : " Initially the BOG and the loop were clearly superior to the vertical, and throughout the 18 months the loop performance was very consistent. The BOG worked well at first. However, over time and especially during the two intervening winter wet seasons,I noticed the BOG signal amplitudes dropping off significantly (-15 dB) and the S/N improvement dropped to no better than the vertical."
This aligns exactly with my findings. When the BOG is installed at the end of October, when the ground is still dry, I notice an advantage in terms of reception. From the moment everything is wet, the vertical performs better.