Do you have the correct entry in sources.list? That points to the /usr/... path and is required, while the one in /etc/... is global:
echo “deb [signed-by=/usr/share/keyrings/syncthing-archive-keyring.gpg] https://apt.syncthing.net/ syncthing stable” | sudo tee /etc/apt/sources.list.d/syncthing.list
Deprecating apt-key because it does the wrong thing instead of adapting it to to the right thing was definitily a super bad decision.