A missing star in the push-button is as a rule a defect and not a feature. The logo is a separately made part glued into a recess; when the adhesive gives way, it is lost. That is easily checked: if the push-button shows a recess, possibly with traces of adhesive, a star originally sat there. A smooth, continuous surface with no recess, by contrast, would point to a factory version.
Listings that advertise such a pen as a rare variant are to be treated with caution as long as there is no evidence of who is supposed to have supplied it that way. No premium for rarity is justified, all the more so since a workshop can replace the star. Before buying at a distance it is advisable to ask for a sharp close-up photograph of the push-button.
Montblanc writing instruments without a star have nonetheless existed: for particular markets, above all for deliveries to Arab countries, the logo was in some cases altered or left off. That practice, though, affected the design as a whole; a pen with the star present on the clip but removed from the push-button does not fit the pattern.