Small clarification: Android 6 has a unified fingerprint API for all phone manufacturers to use, and so apps can then just do one thing and fingerprints work, this appears to be what Monzo have done.
Samsung phones with fingerprint readers and that shipped with Android 6 have this API implemented. e.g. my A3 2017 works fine in Monzo.
It seems some Samsung phones that started on lower Android versions, that have this “Samsung Pass” proprietary fingerprint API (a good thing at the time, because there otherwise wasn’t an API for devs to use), with the OS upgrade to 6 didn’t get official unified API support added in. It’s hard to get info of exactly which phone models got the upgrade but no support for the unified API, all I found was some stackoverflow threads that peter out without useful info.