[Previous reply deleted as it is too verbose and can be seen as inflammatory]
This is common nowadays and is likely not related to the upgrade. 6to4 requires a public IP, an ISP that does not block protocol 41 or 192.88.99.1 traffic (many do), and a route to 192.88.99.0/24 that does not lead to an ISP that blackholes, throttles, or delays traffic. You will get a 2002::/16 address no matter what, if you have a public IPv4. If the traffic is blackholed by your ISP or a transit ISP, you will get no IPv6 connectivity.
In short: the same-time breakage likely resulted from same-time ISP-side changes, not from the OpenWrt upgrade.
I do not recommend using 6to4 at all, as it's deprecated and does not work in practice (incomplete reachability or huge delays) for many users.
If you have a public IPv4 address, 6in4 (e.g., from HE) may be fine. Alternatives include paid tunnels and VPNs, e.g., from https://hoppy.network (WireGuard, based in Chicago, gives /56 of IPv6) or https://swissvpn.net/index.php?lang=en (PPtP, based in Switzerland, limited to 30 Mbps if you order a speed upgrade and 15 Mbps otherwise, gives /64 of IPv6 if you ask for a static IP). Even though PPtP encryption is broken, that's OK for your use case specifically, as the goal is obtaining IPv6 connectivity, not hiding anything from traffic snoopers. As a reminder, we are comparing this with 6to4, which is completely unencrypted.