I can only make a guess that maybe, if the heater is hot, the shower likes the water to flow for a while longer to dissipate the heat as it cools ? Maybe ? But I can't think why it would need longer as it got older. Maybe there's a valve sticking and taking a while to block off ?
There will be a close off valve in the shower with some form of washer. it sound like the washer has worn or has collected residue on it causing it longer to close.
Contact the manufacturers to see if a Service/Repair kit is available.
Just a pack of washers; O-rings etc. That fixes most things. If not, then it's time to consider the age of the unit, and whether it may make more sense to replace it.
thanks, it is several years old and gets used several times a day most days.
Will consider the cost of someone to repair might be cheaper in the long run just to replace.