Bonding of hotpluggable interface fails after un- and re-plug the last device

I want to bond the network interfaces of two USB-tethered android phones connected to a Raspberry Pi 4. So when one person leaves and takes his phone, it "fail-overs" to the second phone without a short interruption you would normally have without bonding. The problem is that the bonding network interface disappears when I remove and replug the last phone (and activate USB tethering again). This is how I set up the bonding interface. I use systemd-networkd: /etc/systemd/network/bond1.netdev

[NetDev]
Name=bond1
Description=Bonds usb network interfaces
Kind=bond

[Bond]
Mode=active-backup
MIIMonitorSec=100

/etc/systemd/network/bond1.network

[Match]
Name=usb*

[Network]
Bond=bond1

/etc/systemd/network/bond1-net.network

[Match]
Name=bond1

[Network]
DHCP=yes

This configuration seems to work as long as at least one usb network interface is there and is not removed/replugged. As soon as I un- and replug (i.e. not upon removal) the last phone and activate USB-tethering the usb0 network interface appears again and bond1 immediatly disappears and I have to restart systemd-networkd to let it reappear again. After removing all bonded usb network interface ip link still lists bond1:

5: bond1:  mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 4e:db:45:d2:25:c0 brd ff:ff:ff:ff:ff:ff

After replugging and enabling usb tethering bond1 is gone and dmesg says:

[ 1345.285781] bond1: (slave usb0): making interface the new active one
[ 1345.286004] bond1: (slave usb0): Enslaving as an active interface with an up link
[ 1346.647947] bond1: (slave usb0): Releasing backup interface
[ 1346.682264] bond1 (unregistering): Released all slaves

So at first it re-adds the usb interface again but then releases all interfaces and unregisters bond1. Then I tried to find a solution and found the following hints: Add these lines to /etc/systemd/network/bond1.netdev

[NetDev]
Name=bond1
Kind=bond
Removable=no # Added

[Bond]
Mode=active-backup
MiimonSec=100
PrimaryReselectPolicy=better # Added
IgnoreCarrierLoss=1 # Added

And add udev rules to manually re-add the usb interfaces:

SUBSYSTEM=="net", ACTION=="add", KERNEL=="usb0", RUN+="/usr/bin/ip link set usb0 master bond1"
SUBSYSTEM=="net", ACTION=="add", KERNEL=="usb1", RUN+="/usr/bin/ip link set usb1 master bond1"

But that does not help or change the behavior in any way. Does anyone know what might help to fix this problem?