The safety lead time works as a buffer to make sure that you receive items in due course.

Or, according to your documentation (https://docs.microsoft.com/en-us/dynamics365/business-central/design-details-planning-parameters#optimize-when-and-how-much-to-reorder): The default safety lead time, on the Manufacturing Setup page, should be set to at least one day. The due date of the demand may be known, but not the due time. The planning schedules backward to meet gross demand, and, if no safety lead time is defined, the goods may arrive too late to meet the demand. The safety lead time on the item card works exactly like this - also for items with replenishment system "prod order".

In practice, you get a solid (and well working) buffer when applying the safety lead time to items that you produce.

However, the nature of a buffer is to give you flexibility. And the nature of a buffer is to get used e.g. to cope with unforeseen events. This means that a buffer is great to get applied when initially planning a production order. However, when replanning/rescheduling it, the buffer might get used (rather than being applied again and again). Currently, Business Central applies the same buffer (aka safety lead time) any time a production order is replanned/refreshed.

Here is what happens: the sales order defines the demand date. You create a prod order from that demand date, the safety lead time (say: 5 days) gets applied and the prod order is planned to start in due course taking into account the safety lead time. You also have a reservation between the sales order and the prod order. All good. Now you have to move that prod order (for whatever reason) by one day. As you initially planned it with a buffer of 5 days, you still finish it before the demand date. However, this change updates the prod order due as it adds the default 5D safety lead time to the new prod order finish date.

As a result, the reservation between sales order and prod order is deleted - although the prod order still finishes in time. Even worse: the user isn't even notified about the loss of the reservation.

My suggestion is to add a flag to the safety lead time called "apply only for initial prod order planning". If this flag is set to "yes", then the safety lead time would be applied when the prod order is created. After that, the user has the buffer and time-flexibility to change the prod order. If we violate the buffer and move the prod order end date past the demand date, BC would continue to work as today. By making this a flag/option, BC would continue to work as is, but users would get the option to work with the safety lead time as a true buffer.