My iPhone (HK version thus dual physical SIM) 12Pro, 12ProMax, and 13ProMax are all with Simba SIM cards. Data Roam is turned on 24/7 ever since local data is shared.
Been crossing the causeway every other week, and all phones auto-connect to Celcom not long after I get into JB. Especially the 12Pro which I rely on for TnG payments right from the shop in BSI, the CIQ building on Woodlands side.
Other SIM card in each phone is an old Malaysian Prepaid, which I switch over for data as and when I need.
Coming from the likes of ST and M1, which I previously had to worry about this and that.
So no, I won't say it's due to the older iPhones. (Have Eight in an iPhone 11Pro, auto-detect though takes a bit longer, and I suspect it's only because Hotlink and U Mobile aren't exactly very active around BSI.)