Apparently the lxd appliance needs to use a custom track, so hack it in.

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.525.47) UNRELEAED; urgency=medium
+
+  * Apparently the lxd appliance needs to use a custom track (4.0) and since
+    model assertions do not accept track names in required-snaps, we need to do
+    this by hand during build. (LP: #1891505)
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Wed, 12 Aug 2020 15:58:19 +0200
+
 livecd-rootfs (2.525.46) bionic; urgency=medium
 
   [ Cody Shepherd ]

live-build/auto/config

@@ -219,6 +219,10 @@ case $IMAGEFORMAT in
 					[ -z "${SUBARCH:-}" ] \
 					&& UBUNTU_IMAGE_ARGS="$UBUNTU_IMAGE_ARGS --image-size 3700M"
 					;;
+				# Some appliances need different snap tracks
+				lxd-core18*)
+					EXTRA_SNAPS="lxd=4.0"
+					;;
 			esac
 			case $SUITE in
 				xenial)