I understood that the museum was, or has been, closed for a refit over a year or so. Either it is not open fully or it might have been decided to not exhibit an entire inventory of vehicles. This happens all the time.
Off course a trust can sell off exhibits to create finance to re invest. Anyone assuming that putting a car into a Museum with, or without, a trust to protect it, or restrict its sale, are deluding themselves. In fact you are transferring responsibility for ultimate disposal to someone else at some future date with the option of a nice little earner on top. That is fine in most cases but not quiet so good when folk do it out of charitable conviction only to then see their donated car sold off a few years later for a higher price. Conversely most donated cars do not come with a stipend or finance to maintain the donated car for the future in a collection, so one does wonder quite what the thought processes are. Cars cost money to exhibit, maintain and store. That has to come from somewhere and how many Isettas does a Museum need. Cash clearly being not as emotional as a classic car, I guess, people get upset if the one is converted to the other to further a collection, though the cash is still a donation. Certain Museums are habitual sellers of donated cars and even go so far as to present an image likely to obtain new bequests and subsidised purchases to later be sold at a profit. I find that morally wrong, but otherwise, that's life.