Manually selected mobile network changes when signal strength drops
It seams to me a problem of Android 6.

I have a similar problem with SONY XPERIA XA.

my behaviour:

SIM1: Manual on Sunrise (CH)
SIM2: Manual on TIM (I)

Reboot the phone:
SIM1: Manual on Sunrise (CH)
SIM2: Automatic (it select the power operator May Swisscom, TIM or Orange).

So afer any reboot you should select manualy the network (if available!).

Don't know how to create a BUG request for android.

I allredy tyied to contact SONY, but no way!

regards
Stefano