Jethro's excellent website notes that TUI withdrew a 738 (TAWR) in Feb 23, and it has now been registered to Sun Country in the US. A further example has recently gone to Transavia.
Although there may be another explanation (like lease ending) it does seem odd to be withdrawing aircraft in the early months of the year only to replace them with leased equipment weeks later.
It just adds to the impression that TUI have lost the plot!