How can I figure out why my network configuration is simply ignored?

I have been running Debian 10 a.k.a. Buster without issues. My problems started when all of a sudden, network connectivity dropped. At that point, NetworkManager was active. I stopped and disabled the service to exclude its interference. This PC runs KVM VMs in a bridged configuration:

$ sudo brctl show
bridge name	bridge id       STP     enabled	interfaces
br1     8000.7446a0a0d47b   no      eno1
                                    vnet0
                                    vnet1
                                    vnet2
                                    vnet3

My interfaces file looks like this (leaving out a few comments):

$ cat /etc/network/interfaces
(...)
# The loopback network interface
auto lo
iface lo inet loopback

#auto eno1
#iface eno1 inet manual

auto br1
iface br1 inet manual
bridge_ports eno1
address 192.168.1.27
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 192.168.1.16, 1.1.1.1

When I run systemctl restart networking, or ifdown br1; ifup br1, br1 ends up with no IP address, both eno1 and br1 are down, and there is no default route. Removing the hash signs on the eno1 lines has no effect. In my desperation, I added this line to the interfaces file: this line is illegal. When it comes **before** the loopback block, it causes an expected error message:

ifup: /etc/network/interfaces:7: misplaced option
ifup: couldn't read interfaces file "/etc/network/interfaces"

When it comes **after** it, it doesn't. It's as if ifupdown stopped reading after the loopback configuration. But putting the loopback block at the end of the file still doesn't configure br1. I haven't detected any non-printable characters or otherwise strange characters in the file. To cross the T's, here is the journal:

$ sudo journalctl -u -e networking
(...)
-- Reboot --
May 21 09:29:58 jupiter systemd: Starting Raise network interfaces...
May 21 09:29:59 jupiter ifup: Waiting for br1 to get ready (MAXWAIT is 32 seconds).
May 21 09:30:00 jupiter systemd: Started Raise network interfaces.

And setting STP on the bridge doesn't change the behaviour. How about ifup?

# type ifup
ifup is /usr/sbin/ifup
# file /usr/sbin/ifup
/usr/sbin/ifup: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=9ac8619bfa91d2b8901228595fa390826c9682c7, stripped

**How can I figure out why my network configuration is simply ignored?**