I'm in the UK (Gloucestershire) and have a mature sycamore (Acer pseudoplatanus) at the edge of my garden, which was planted by my neighbours between 40 and 60 years ago (the survivor doesn't remember when her husband planted it on the boundary of the field outside their garden 'on a whim').
16 years ago, when I moved into a house newly built on the field next door, I had the sycamore pollarded (it was far too big to contemplate otherwise). Since then, it has been pruned by professional arboriculturists every 2-3 years in winter to keep the height under control i.e. to less than the height of a two storey house. This allows my neighbours to use the greenhouses they sited under the tree they planted, but allows me to have useful shade at the side of my back garden -- and I've planted the borders to allow for this.
Recently however (last 4-5 years), I've observed a section (always the same section every year) of the leaves browning at the edges and eventually falling prematurely. The rest of the tree seems healthy, but the effect is unsightly and gives me concerns about the overall health of the tree.
What could be causing this, and is it capable of remedy, or do I need to sacrifice the important shade and replant my borders?