can't think why they would react that much to warmer water?
are you sure your heater/temp gauge are working properly?
ie your temp' is accurate
having it set that high and you will probably find you need to take more care of the water, and make sure plenty of water movement, maybe add some air stones etc,
obviously it isn't going to harm them otherwise you wouldn't be able to up the temp' when sick, but pleco's deffo prefer lower temps'
some will even live in cold water, keep your eye on it to make sure it's feeding and acting normally, if you havn't got any put some bogwood in for the plec'
raising the temp' may effect the way it feeds (or not) so as i say try and keep an eye on it