Missing slots calculation

I'd suggest to add helper column to group frames, something like

=N($I1) + --([@Frame] <> $A1)

Next we could GROUPBY by them

=LET(
    slots, {3,8,13,18},
    frames, GROUPBY(
        HSTACK( Table1[[#All],[Group]],Table1[[#All],[Frame]] ),
        VSTACK("Missed", Table1[Slot] ),
        LAMBDA(a, ARRAYTOTEXT( TOROW( slots / ISNA( XMATCH( slots,a ) ),2 ) ) ),
        3  ),
    dropped, DROP(frames,,1),
    FILTER( dropped, NOT( ISERROR( CHOOSECOLS(dropped,2) ) ) )
)

Performance is not very good. Perhaps we may optimize above, but don't thing that dramatically.

By the way, in Power Query version is the bug, very last frame is not calculated correctly. Since you prefer formula I didn't fix it.