I have a feeling that TUI may have already made the decision to not include the Max aircraft in their summer schedules, As it has been noticed in the last week a number of flights to Turkey that were due to be on Tui aircraft has now been moved to Freebird airlines often doing two rotations a day for Tui from DLM and AYT.
Further to this there appears to have been some capacity cutbacks made for example on Weds from MAN and LGW to PFO they were due to have Dreamliner out in the morning and then a 738 or Max in the late afternoon instead there will just be the Dreamliner flight which will just push the holiday costs up and expect that they start using more ad hoc capacity with TCX and EZY