It might be answer that you say the subscribers do not latch in automatic mode. If attempting to latch to the new (roam) network it could see these subs handsets as barred or stolen or not paid contract fees etc. This is potentially possible because of SIM Card Registration failed – which the subs authentication failed in that network.
This could be a commerical decision to force your HLR to agree to a roaming agreement (thus roaming traffic) on new network (new networks cost money, need traffic theirs or someone elses to pay for network). If other network is using (anti-) steering of roaming it could be they use subs handsets in auto roaming mode, lock auto selection to their network – does roaming network send welcome SMS maybe? Ask subs to check if they get SMS and is it still saved.
This maybe because you say when selected in manual mode subs have no problem getting back to your network (their home network). the auto mode is bypassed.
Maybe look at make/model of handsets or handsets with particular operating platform (symbian, etc) or where 2G or 3G handsets.
Also contact other network to find out how they organise roaming registration.
Could be in that border area the other network change some in BSS or something. I guess at that because I dont know your network or other network. So just give ideas.