Corbyn's non-attendance at the Festival of Remembrance (which all other opposition party leader also seemed not to attend) may well have been down to the fact that he is seventy years old I believe and is also leading a large political party in the middle of a General Election campaign. Therefore, energy, stamina and a pact diary may well have played a large consideration in the decision not to attend.
Johnson was there in his role as Prime Minister but he has the organisational support of Downing Street for his diary and is half Corbyn's age.
So, we now have another "let's slag off Corbyn thread" with the usual extream right-wing zealots populating it.