@flebourse thanks to give the indication where counter can be retrieved.
Thus I performed a new test by triggering data reset of SIM2 (daily + hour) and as @flebourse mentioned, this couter is properly reset. Then I perfomed a SIM switch to SIM1 (by applying anoter profile) then come back again to SIM2 (by applying again previous profile) and then as @flebourse observed I confirm that counter retrieves a previous value before reset (but not exactly last one before reset either). Modifying date of automatic reset also result in counter alteration:
root@Teltonika-RUTX09:~# cat /proc/net/xt_quota/mob1s2a1
94848523295
After the automatic reset time => counter was reset:
root@Teltonika-RUTX09:~# cat /proc/net/xt_quota/mob1s2a1
104795394705
Some times later => counter normaly decreased
root@Teltonika-RUTX09:~# cat /proc/net/xt_quota/mob1s2a1
104723285908
Right after, performed 2 SIM switch SIM2->SIM1->SIM2 and then => counter restored to an inconsistent value:
root@Teltonika-RUTX09:~# cat /proc/net/xt_quota/mob1s2a1
95799944725
Then I restore my original settings for data limit reset and then => still another value from nowhere !!!
root@Teltonika-RUTX09:~# cat /proc/net/xt_quota/mob1s2a1
86786101182
Indeed many way to reproduce the issue and never work (I wonder how feature was tested :-( ...)