Galarian-Darmanitan using Max Strike if he has Choice Scarf equipped and you Dynamax;
https://gyazo.com/ddc37c58a8573360b02f6019a39b5c46

Thank you for reporting this I was able to confirm this and will go ahead and mark it to be moved to the internal tracker.

Clarification for internal tracker:
When Dynamaxing using a Galarian Darmanitan that's holding a Choice Scarf it will use the intended max move on the turn it Dynamaxed but on each consecutive turn when you click on "Fight" to select the move it will default to Max Strike and conduct battle and once Darmanitan reverts to normal each time you select on "Fight" it will default to Struggle and go on to conduct the battle.
On further testing on other Pokemon with the Choice Scarf the battle conducts as intended while Dynamaxed and once it's reverted.
Some more testing has revealed that only a Galarain Darmanitan with Gorilla Tactics and a Choice Scarf is causing this bug. If it has Zen Mode then it will work as intended.

Forwarded to our internal tracker.

Hi,

This issue should have been resolved for version 8.3.7, let me know if you have any further issues. Thank you for your report!