Chinese elm *loves it hot - mine stay full-sun in 100+F weather, and have zero issues. If all is well, such as soil, fert, and watering, there would be nno issue with what you've said so far. We need a bit more info here. I am assuming you are in S Africa - Cape Town?? No idea of weather there, but should be within elm limits, IMO, as they are very tolerant of temp-ranges.
What soil, when last repotted, how do you water it (regular on schedule or as-needed only) - and do you have any pics? I am VERY interested in the soil used as first point
And you say this is a tree of about 34yrs age? If it is in a tight root-restricting bonsai pot, definitely need to know about repotting efforts/times. How long have *you* owned it, and how many years have you successfully kept it healthy and vigorous? No insult meant, just curious as to your experience and overall understanding of pot-culture/growing methods.
Let's start there - a small story of your overall history/methods of the last two frowing seasons (and has it been dormant at all over the last three calendar years???
)... I suspect that the tree is very weak from lack of dormancy, but just a guess (more info, please, on dormancy methods used, if any)
Fwiw, intense-sun will not harm elm leaves that are accustomed/acclimated to it - they can possibly burn if a full-time indoors-tree is placed suddenly out into scalding sun without gradual acclimation, but any/all regrowth will adapt as it extends, so there is an end-point to leaf-burn (make sense?). When new buds fail to extend/live, something else is most definitely the issue(s)
Is it possible to gently/easily lift rootball a bit to see if there is *any* growing space left within soil? Just a quick peek is all I mean - and if there's any resistance in lifting tree, forgo that...do no harm
A seriously root-bound tree can show symptoms such as you describe (but so can many other things, too, of course).