MTU is reset despite user definition

I found some websites were not loading when using my iMac and Macbook Air wifi. The problem was the MTU size.

When I navigate to System settings, Network, Wifi, Details and then Hardware the MTU can be changed, so I set to Manually and 1500.

Although when I look at it again immediately after making this change the OS has reset this to 'Automatically'.

Setting to Manually and Custom value I can set a custom value, but if I set Standard (1500) this again is ignored and the 'Automatic' setting re-applied.

When I set Custom, and a specific value this is accepted but I want to select Manually / Standard(1500), not 1436 as this value causes problems of its own.

There's a large number of posts regarding the failure of the MTU to properly set going back to Snow Leopard. This is a defect. A bug that's been present for years.

I appreciate it is not glamorous to fix bugs, but this would be far more useful than the pointless and intrusive 'tips' bloatware nonsense foisted on users, the sign in to use Messages (which I've never opened and would like rid of), or the unnecessary UI fiddling.