I just saw that an 80 character per line limit applies to entries in /boot/config.txt. I suppose that this holds true for /boot/userconfig.txt, too. According to the linked readme any characters exceeding the limit will be ignored.
The entry beginning with “dtoverlay=ads7846…” has more than 80 characters. So I have to reason that the parameter keep_vref_on=0 is the last one that is complete and gets processed whereas all following parameters get dropped, so amongst others xohms=150.
To circumvent the character limit split the “dtoverlay=ads7846…” like this:
dtoverlay=ads7846
dtparam=cs=1
dtparam=penirq=25
dtparam=penirq_pull=2
dtparam=speed=50000
dtparam=swapxy=0
dtparam=pmax=255
dtparam=xohms=150
dtparam=xmin=200
dtparam=xmax=3900
dtparam=ymin=200
dtparam=ymax=3900
I removed the parameter keep_vref_on=0 as this seems not be valid for the ads7846 overlay (readme). I have no clue why Waveshare has that parameter in its config-5.txt file I have linked above…