Try look in developer options got one is "Mobile data always on". Maybe got accidentally turn in on?
Mine really so far working alright. Auto switching. Only in condition where the wifi has no connectivity of which it will also prompt me that the wifi has no internet connection.
But I've grown aware of checking the icons and disabling data before doing any major data use though so not really an issue for me.
Aiya easiest way to solve, get unlimited data plan
The new Singtel subsidiary GoMo sim only package 20GB 200 mins for $20 looking good actually. For me due to work need more mins not sure whether they going to offer mins add on. Now only can add $10 for 10GB 100mins. Now on starhub with unlimited weekend using around 19-20gb each month including 9gb weekday around 10-11gb weekend. Just totally don't like starhub network. It's not as reliable and stable as singtel in my personal opinion.