shared buffers

hello,

got something strange to me:

Same db ie. same data, around 1.2TB,one on pg13, one on pg16

same 16 GB of shared_buffers,

I am the single user.

both have track_io_timing on

on pg13, if I run a big request with explain (analyze,buffers), 

I see around 6 GB read

if I do rerun the very same request, no more read(s), all data in the shared buffers cache. fine

If I check with pg_buffercache what's in it, I see the biggest tables of my request within the biggest users (in number of blocks used). All this is fine.

next, if I do the very same on the pg16 machine, whatever the number of times I rerun the explain (analyze, buffers) of the same request, each time, the explain shows the same volume of reads. again and again.

If I check with pg_buffercache, the set of objects stay the same, WITHOUT the objects of my request, just like if those objects where sticky.

any idea ?

thanks