Observed:

# mkinitrd
...
dracut: *** Including module: plymouth ***
/usr/lib/plymouth/plymouth-populate-initrd: line 450: +: command not found
dracut: *** Including module: obs ***


Package at fault:
plymouth-dracut-0.9.5~git20210406.e554475-150400.3.5.1.noarch

There really is a stray + in there:

...
PLYMOUTH_TitleFont_PATH=$(fc-list |grep $(fc-match "$(echo $(grep "^TitleFont
*= *" ${PLYMOUTH_SYSROOT}${PLYMOUTH_DATADIR}/plymouth/them
+
[ -f "${PLYMOUTH_Font_PATH}" ] && inst ${PLYMOUTH_Font_PATH} $INITRDDIR
[ -f "${PLYMOUTH_TitleFont_PATH}" ] && inst ${PLYMOUTH_TitleFont_PATH}
$INITRDDIR
...