In a flow, rehearsal numbers show only blank boxes when bar number is selected as the sequence type

[Lillie_Harris]

If the flow in which this occurs includes some independent time signatures, then I believe this is a known limitation.

Empty rehearsal mark?

This will, I suspect, be caused by using bar numbers for rehearsal marks and where you also have local time signatures. Rehearsal marks and bar numbers in general can only use the global information for time signatures to determine their values.