I can confirm, that this patch from Adrian Bunk indeed fixes the problem:
Could you please apply this patch and trigger a rebuild of boost1.90?
Thanks, Helge
--- boost1.90-1.90.0/debian/rules 2026-04-01 07:53:39.000000000 +0300
+++ boost1.90-1.90.0/debian/rules 2026-04-01 07:53:39.000000000 +0300
@@ -174,7 +174,7 @@
bbv2dir = $(CURDIR)/tools/build
# With --ignore-site-config, can probably drop Build-Conflicts on boost-build.
-JAM = $(b2) $(JOBS) -q -d2 $(JAM_OPT) --layout=system --ignore-site-config
--user-config=$(CURDIR)/user-config.jam debug-symbols=on
+JAM = $(b2) $(JOBS) -q -d2 $(JAM_OPT) --layout=system --ignore-site-config
--user-config=$(CURDIR)/user-config.jam debug-symbols=on
address-model=$(DEB_TARGET_ARCH_BITS)
