Plan regression from 12 to 13

Hi,
I have a query (generated by RequestTracker) that has regressed after
upgrading from postgresql 12 to 13. The strange thing is that decreasing
the effective_cache_size causes postgresql 13 to generate an acceptable
plan.
My setup is: 1 VM, 4 cores, 16 GB RAM (server has around 10 GB for
filesystem cache)
shared_buffers 2.4 GB

With effective_cache_size 8G, I have the following plan:
https://explain.depesz.com/s/lIcV (the run time is after several tries,
with most of the data in memory; the initial run reading from disk is
around 10 times slower)
If I decrease the effective_cache_size to '2GB' or less, I get a much
better plan: https://explain.depesz.com/s/Z6C7W
The old VM running postgresql 12 (with a somewhat smaller database) always
generate a good plan,regardless of the effective_cache_size :
https://explain.depesz.com/s/iawn

Are there any defaults that have changed from 12 to 13 that would explain
the different plans?

Thanks,
Radu