Help : IPSEC and cellular modem produce corrupted SEC packet

Hi list,
We had some strange problem : Linux 6.12.16 , ARM64 armada-3720 IPSEC
in tunnel mode.
Userspace is strongswan.
It works ok for wired transmission and also for cellular on kernel 4.19
 SEC layer looks OK in tcpdump of wwan0.
We had offload EP97 engine, driver crypto-safexcel is loaded but ipsec
is not configured to make use of it
But receiver got corrupted SEC layer packets
We tried several cellular modems , same  problem
So the problem I conclude is in kernel

Now the MAGIC
On a hunch I disabled   INET_ESP_OFFLOAD and XFRM_OFFLOAD  in kernel config .
It fixed the problem.
Why ?

I am far from expert in kernel networking, not even close. Can u
please advice me in what direction I should dig ?

Br.
Lev