This has been posted by TUI on Facebook in response to customer comments:
”Hi, due to various factors causing delays we felt the impact to customers holiday was too great and took the difficult decision to cancel some flights. A full refund will be given ASAP, we understand how disappointing this is and we do apologies for any inconvenience caused. ^Dan”
Their Facebook/Instagram posts are riddled with people complaining about recent delays or stuck in airports with no info. Also people considering cancelling and switching to Jet2 due to news of their recent performance spreading. I truly hope things improve as this will not help their reputation.