You must be | It must be

In the English Grammar Profile, B1 point 69 in the category of MODALITY is defined:

DEDUCTIONS AND CONCLUSIONS: ‘must’ to guess and deduce, most commonly with ‘it must be’, or ‘you must be’.

*We cannot ensure automatically that this form will point to the usage.  We have deleted some ngrams that are obviously not the usage. Both examples in the EGP are followed by an adjective.  The surest of these are adjectives with negative meaning.

A search in iWeb for:

it|you must be _JJ

3 YOU MUST BE AWARE 2142
4 IT MUST BE TRUE 1798
5 YOU MUST BE SURE 1384
6 IT MUST BE GOOD 1096
7 IT MUST BE POSSIBLE 1094

8 IT MUST BE HARD 750

BBC

George Floyd death: How US police are trying to win back trust

It must be hard being a police officer in America, especially after the death of George Floyd.

11 IT MUST BE LIKE 600

14 IT MUST BE DIFFICULT 539

15 YOU MUST BE FAMILIAR 530
16 IT MUST BE NICE 466
17 YOU MUST BE COMFORTABLE 439
18 YOU MUST BE ELIGIBLE 353
19 IT MUST BE EASY 347

21 IT MUST BE TOUGH 266

22 IT MUST BE UNIQUE 264
23 IT MUST BE RIGHT 260
24 IT MUST BE GREAT 259
25 YOU MUST BE CONFIDENT 243
26 IT MUST BE OK 223

27 IT MUST BE FRUSTRATING 200
28 YOU MUST BE CRAZY 193

29 IT MUST BE SAFE 162
30 IT MUST BE IMPORTANT 161
31 YOU MUST BE CERTAIN 156
32 YOU MUST BE NEW 148

37 IT MUST BE AVAILABLE 146
38 IT MUST BE OKAY 145

39 IT MUST BE BAD 145

40 IT MUST BE BETTER 137
41 IT MUST BE OBVIOUS 134
42 IT MUST BE FREE 124

43 IT MUST BE WRONG 118

45 YOU MUST BE HAPPY 107
46 IT MUST BE DIFFERENT 105
47 YOU MUST BE GOOD 104