Sunwing are also having a very bad time in Canada with their own booked pax - cancelling flights due to their MAX grounded, leaving passengers with no booking to get home, or go on holiday - Seems folk there just have to accept a full refund, or wait 9 days at their OWN expense for next flight...
Thankfully we (the pax) still usually have in UK the right to duty of care (hotac/meals/taxi etc) under EU 261 + the delay compensation if applicable and meets the criria which TUI Airways has to foot the bills for. (not TUI Holidays)
Under their contract with Sunwing they may have a claw back clause but we are likely not to be privy to that.
The 737 MAX debacle of delays and cancellations now is “operational circumstances”
The European Court of Justice (ECJ) has repeatedly stated that those don’t qualify as “extraordinary circumstances” so pax will have protected rights of care and rerouting etc for delays, OR any flight cancellations made within 2 weeks of departure.
More than 2 weeks notice of a cancellation you only get a full refund - or the choice to re book when seats are available on the next flight.
In TUI's case their flights are sold as Package Holidays and thus are not usually cancelled - and from the posts above we see already that very early on in the summer season some HUGE 24 hour delays affecting their flights mainly at outstations.