It was due to thinko in iterating through keys set - we were using key
ordinals instead of keys themselvs. Now it find such bugs:

=== RUN   TestΔBTailAllStructs/Update/T1,3/B0:d-B1:a-B3:d,4:g→T1,4/B0:e-B1:a-B4:c
    δbtail_test.go:839: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0}]
                want: map[000000000000000c:{pffffffffffffffff c3} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0} 0000000000000620:{p000000000000000c c0}]
    δbtail_test.go:708: T1,3/B0:d-B1:a-B3:d,4:g -> T1,4/B0:e-B1:a-B4:c  ; tracked={0 1} :

        d12:  map[0:{d e} 3:{d ø} 4:{g c}]
        δTok: map[0:{d e} 3:{d ø} 4:{g c}]
        δT:   map[0:{d e}]

        δZ:               {000000000000000c 0000000000000618 0000000000000620}
        Tracked^δZ:       {0 1}
        kadj[Tracked^δZ]: {0 1 3 4 9223372036854775807}
        kadj: map[0:{0} 1:{1 3 4 9223372036854775807} 3:{1 3 4 9223372036854775807} 4:{1 3 4 9223372036854775807} 9223372036854775807:{1 3 4 9223372036854775807}]

        δtkeycov wrong:
        have: {}
        want: {[3,∞)}
        δT    ∌  δTok[3]
        δT    ∌  δTok[4]