Incomprehensible problem about sessions

Thank you for your answer, given that my environments are in linux containers my software configuration has not been modified.

Is there any error message?
>> No error message, no log error.

What should it do:
>> echo data work

what does it actually do:
>> echo data don't show anything. is empty

The server I took is 10x faster than the previous one, even if it does not seem possible to me, is it possible that the variable is erased before being displayed? I also tried to go through a buffer variable, but I have the same result, ex:
tmpvar = $ this-> session-> ajax_footer_load;
echo tmpvar;

mark_as_temp works and displays my data, but it's still quite strange.