Stealth Units disengaging from melee combat

Scenario

Lvl 4 NB's spotted enemy Lvl 3 Sub.

Set to Patrol over them. Into the path of the Subs I moved a Lvl 2 DD to keep them from moving out of NB patrol range and, they made contact and Sub vs DD naval battle began and they started Ticking.

I check back 20 minutes later and the Sub disengaged from the melee fight with the DD, and had to relocate NB's...

Why would they disengage.

Has something changed in melee combat with stealth units of higher levels?

I was pretty sure that once engaged in melee combat, units lost their stealth ability (this has been the way of things for years).

What could I be missing here?