Package: eudev
Version: 3.2.7-6

This bug has been described here; too:
https://bugs.gentoo.org/show_bug.cgi?id=739268

tl;dr:
messages saying "Aug 27 16:30:50 MiniHTPC2 udevd[2291]: failed to execute
'/usr/lib/udev/${exec_prefix}/bin/udevadm' '${exec_prefix}/bin/udevadm
trigger -s block -p ID_BTRFS_READY=0': No such file or directory" do
appear in dmesg log (similar on my system, except for the host name)
To fix, ${exec_prefix} needs to be set in the build scripts

I fixed it locally; until then, the config files were original.

Effective local patch:
--- /lib/udev/rules.d/64-btrfs.rules~   2019-02-09 13:22:24.000000000 +0100
+++ /lib/udev/rules.d/64-btrfs.rules    2020-11-16 15:02:07.720000000 +0100
@@ -11,6 +11,6 @@ IMPORT{builtin}="btrfs ready $devnode"
 ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"

# reconsider pending devices in case when multidevice volume awaits
-ENV{ID_BTRFS_READY}=="1", RUN+="${exec_prefix}/bin/udevadm trigger -s block -p ID_BTRFS_READY=0"
+ENV{ID_BTRFS_READY}=="1", RUN+="/bin/udevadm trigger -s block -p ID_BTRFS_READY=0"

LABEL="btrfs_end"

Have a nice day; 
    Bodo Eggert