The problem was that instructions sp_instr_cursor_copy_struct and
sp_instr_copen uses the same lex, adding and removing "tail" of
prelocked tables and forgetting that tail of all tables is kept in
LEX::query_tables_last. If the LEX used only by one instruction
or the query do not have prelocked tables it is not important.
But to work correctly in all cases LEX::query_tables_last should
be reset to make new tables added in the correct list (after last
table in the LEX instead after last table of the prelocking "tail"
which was cut).