After a long time running without issues, the last hope mote (da80) started to print this error:
ERROR da80 [IEEE802154E] wrong state 6 in startSlot, at slotOffset X
This last some time causing up to 15 de-synchs. After a while it recovered and continued normally without any other de-synch. My guess is that this has been caused by this slight and sudden increment of dagrank causing eventually a shift of parents and making motes enter in a parent-parent loop.