M20 bro.
The reason arms come loose on ALL Steps axles is because unlike a proper HTII system on a normal bike the arms are not preloaded against anything. This means as they flex the arm moves slightly on the splines. eventually "walking" outwards on the axle. leaving the plastic cap no chance in holding it's threads. Even with correct crank arm pinch bolt torque
An oversight by Shimano's desingers/engineers for sure. But completely solvable by simply using metal pre-load caps.
I worked out the cause and solution over 4 years ago after my E8000 crank came off mid ride (early on in it's life)
Just buy two of these.
For the SHIMANO HTII models listed below Crank Arm Fixing Bolt For Most MTB Shimano HollowTech II Cranksets. M20 Pretension bolt for shimano Hollowtech II cranksets only. Deore, XT, SLX, Saint & Zee (Mtb) Amongst others.
www.ebay.co.uk
they tighten with a 10mm hex - and you can torque the bolts a little higher than the plastic caps. but don't go crazy with a long 10mm. tighten your crank arm pinchbolts to equal torque as normal.
Shimano crank arms will actually pinch down on the outer portion of the new caps and stop them from unwinding. previously I recommended loc tite on the threads but because of this it's not entirely neccessary.
I've done 30000+ miles on loads of Shimano motors over the last 4 years using these and never had another issue with them coming loose. But still come across riders with various Shimano motors who've lost an arm while riding.