It looks like the UI font should have (but no longer has) different feature code to the non-UI font. In particular, deep conjuncts should be squished and lower u-vowel marks should be replaced with UI variants:
$ shape ~/Downloads/android_fonts-master/api_level/30/NotoSansBengaliUI-Regular.otf 'কফ্রুড্রুম্রুদ্বুদ্বৃট্টুক'
[kabeng=0+807|pharasquishbeng=1+877|uvowelsignvattuUIbeng=1@-221,0+0|ddarasquishbeng=5+700|uvowelsignvattuUIbeng=5@-5,0+0|marasquishbeng=9+684|uvowelsignvattuUIbeng=9@-4,0+0|dabasquishbeng=13+633|uvowelsignvattuUIbeng=13@1,0+0|dabasquishbeng=17+633|rvocalicvowelsignvattuUIbeng=17@1,0+0|ttattasquishbeng=21+652|uvowelsignvattuUIbeng=21@-72,0+0|kabeng=25+807]
(But they are not been squished in current UI builds)
$ shape variable_ttf/NotoSansBengaliUI-VF.ttf 'কফ্রুড্রুম্রুদ্বুদ্বৃট্টুক'
[kabeng=0+807|pharabeng=1+899|uvowelsignlowbeng=1@-221,0+0|ddarabeng=5+712|uvowelsignlowerbeng=5@-4,0+0|marabeng=9+658|uvowelsignlowbeng=9@-4,74+0|dababeng=13+617|uvowelsignlowbeng=13+0|dababeng=17+617|rvocalicvowelsignlowbeng=17+0|ttattabeng=21+648|uvowelsignlowbeng=21@-111,0+0|kabeng=25+807]
Additionally, some conjuncts should not be formed in the UI build if they end up too deep:
$ shape ~/Downloads/android_fonts-master/api_level/30/NotoSansBengaliUI-Regular.otf 'কহ্লহ্লু'
[kabeng=0+807|halabeng=1+623|hahalfbeng=4+530|labeng=4+731|uvowelsignbeng=4+0]
(But they are still being formed in current UI builds)
$ shape variable_ttf/NotoSansBengaliUI-VF.ttf 'কহ্লহ্লু'
[kabeng=0+807|halabeng=1+623|halabeng=4+623|uvowelsignlowerbeng=4@-41,0+0]
It looks like the UI font should have (but no longer has) different feature code to the non-UI font. In particular, deep conjuncts should be squished and lower u-vowel marks should be replaced with UI variants:
(But they are not been squished in current UI builds)
Additionally, some conjuncts should not be formed in the UI build if they end up too deep:
(But they are still being formed in current UI builds)